Copy/paste can have its place in the game, that being to fill gaps where a unique vehicle cannot be added, but the problem is that you often completely fail to consider what makes the most sense to add in terms of historical authenticity for certain tech trees, keeping tech trees as their own unique experiences, and not making gameplay incredibly repetitive.
The problem is the copy/paste, whether you intend to add a unique vehicle to fill that same gap later on down the line, still usually hurts the nation said vehicle is being put into. You brought up the Leopard 1A5BE as an example of a sort of stop gap intended to fill the gap between the AMX-30s and the MSC until the AMX-30C2 (which is apparently planned, awesome) can be finished and added. Well, that means that players going through the French tech tree now have to grind through a German tank in the French tech tree, for no other reason than you felt (somewhat justifiably given that gap) that this is a massive gap that needs to be filled…so what happens when the AMX-30C2 DOES come? You don’t remove copy/paste vehicles when the unique ones are ready, so that then leaves people having to grind through a tech tree full of copy/pastes that they don’t want (and in many cases already have in other trees) in order to get to the unique vehicles they are playing that tech tree for.
The same can be said for the BMP-1 when it was first added to the German tree. It helped fill a massive gap in the light tank line within that tree, but only a few updates later we got the Marder 1 at practically the same BR (followed later by the 1A3 at exactly the same BR). Now there’s a random BMP-1 that is completely unrelated to anything sitting in the German tech tree for no reason that players going through the Russian tech tree will encounter that they really shouldn’t, as that’s an east German vehicle that they should be fighting alongside rather than against them. A massive issue with copy/paste you failed to address is that it is not fun having to fight your own vehicle all the time, which is what happens when you remove the match making system that keeps team compositions relatively historical (ie, avoids stuff like having America fighting Britain or WWII Germany being teamed up with Israel) and proceed to copy/paste so many vehicles into other tech trees. If I’m playing America, I want to be fighting Panzer IVs, Tigers, Panthers, etc, and yet I fight Comets, Cromwells, T-34s, and so many Shermans.
While on the subject of the German tech tree, you’ve just mentioned that the M44 and M55 were added to fill the gap in the German tech tree while the Hummel was being developed. What gap? The German tech tree has so many tank destroyers in the 3.7-4.3 BR bracket, and the M44 and M55 as cold war America howitzers did nothing to satiate people waiting for the Hummel, which is a WWII German howitzer. If anything it only made people even more frustrated that it wasn’t in the game yet, because we were seemingly being taunted with two random American SPHs that no one wanted in the German tech tree, and that the German tech tree did not at all need! I think the M44 and M55 are fun vehicles, but any feeling of uniqueness I got from playing that was nonexistent the moment they came to the game since it seemed that virtually everyone had access to them.
You also mentioned that you don’t want to lock unique vehicles behind a paywall, and yet you very often fail to add a unique version of a tank you are copy/pasting into a tech tree that might need it when you absolutely could. As an example, take 3.7 Italy. 3.7 Italy lacked a good medium tank for several years, as the P40’s poor firepower means it massively struggles in uptiers. Italy’s 3.7 lineup still primarily consists of unique Italian vehicles from when they were still on the side of the axis, so surely a Panzer IV would make the most sense, right? Nope, that’s a premium, but at least we got a Panzer III M that they didn’t need and doesn’t fill that gap. Okay, so I guess we’re adding allied vehicles then, so it’s probably going to be a Sherman. The only Sherman in the Italian tech tree at that point was the IC Firefly, but at least it was a composite hull variant of the vehicle unique to the Italian tech tree! Surely it would make the most sense to add a composite 75 Sherman to the Italian tech tree to fill that 3.7 gap, right? Nope, the composite 75 Sherman is ALSO locked behind a paywall, and instead we got the same M4A4 Sherman with no gameplay or visual differences between the one in the French tech tree (that they really didn’t need seeing as they already had the M4A1), and the M4A4 in the Chinese tech tree that is tbh perfectly fine because it actually has a reason to be there.
Also look to the French tech tree, which has a massive medium tank gap between 5.0 and 6.7/7.0. Ignoring the fact that I feel the M4A4 (SA50) should be 4.7 rather than 5.0, this is a HUGE and very important gap that has yet to be addressed. I expected that the Panther would likely be used to fill this gap, as France operated a decent number of them post war, practically wrote the book on what we know about that tank’s performance today, and the vehicle had a noticeable impact on immediate post war French designs. The 75’s performance was liked so much that France developed the SA50 for the AMX-13, and so many of those immediate post war medium and heavy tank designs featured overlapping roadwheels unique to mid/late war German designs. Adding the Panther to help fill this medium tank gap would be a welcome addition…and yet it was locked behind a paywall. Okay, well with Israeli vehicles being added as event vehicles and premiums, maybe a production version of the M-50 Sherman will be added. France already has the prototype at 5.0, so it’s not hard to imagine how fun it would be to play with slightly improved armor, better suspension, a better engine, smoke grenades, and a roof mounted .50. Nope, because instead Israel was added as their own tech tree that consists almost entirely of modified and even copy/paste M48s, M60s, Centurions, and a handful of other vehicles, and even they still don’t have an M-50! Instead, after years of waiting, French players were finally treated to…an M26 at 6.7. A copy/paste American tank that doesn’t do anything to effectively fill this gap seeing as its an overtiered 6.3 medium tank that only serves to fill the shoes of the AMX M4 after it got moved up, along with the rest of the 6.7 lineup. Why an M26 but NOT the Panther? The Panther would have made more sense to be in the tech tree and would have more effectively filled this gap, but instead we get the same tank we can already play in the American and Italian tech trees?
I know I’m rambling at this point, but saying that you use copy/paste to fill gaps just isn’t good enough. Copy/paste vehicles should be added only when necessary, and yet they are frequently added without reason, or as a stop gap that will hurt the tech tree for years to come even though a unique vehicle is already in development. Their impact on gameplay variety (which you talk about all the time when refusing to decompress BRs) should be very heavily considered as it just is not fun fighting your own tanks all the time, or the same tanks no matter what nations you are teamed up with, and yet it obviously isn’t considered given how many Shermans, Stuarts, T-34s, P-47s, etc. there are right now, with the most obvious example being the M44 and M55. A serious attempt should be made to make their models look at least somewhat distinct from their counterparts in other tech trees, whether that mean adding a unique variant rather than straight copy/paste, or modifying the existing variant to have a model that is at least somewhat visually distinct from the vehicle(s) already in game. Take for example the Sherman II, the Rochev, or the previously mentioned composite Firefly. You still occasionally do this (I strongly disagree with the decision to add the Leopard 1A5BE to France or an Italian Leopard 1A2, but they both look great, and I appreciate that France and Italy both have unique vehicles for their quad .50 SPAAs), but for every one of those we seemingly get two instances of vehicles like the M44, the M55, the M4A4, the M18 (why is the M64 an event vehicle instead of a tech tree one again?), the M109s, the Stingray (why does the US tech tree have the Stingray and not the XM8 again?), the M24, the M10, etc.
I’m not saying copy/paste doesn’t have its place, but the way it’s being done in War Thunder right now, and the way it’s been done for years, is frankly unacceptable, lazy, and absurd. There is a way for copy/paste to be done well, and what you are doing right now just isn’t it.