You can’t just mindlessly put any vehocle anywhere or folder it with any other. An SPAA line for example is an SPAA line, so you won’t see an MBT there suddenly.
You also seem to severely underestimate just how many vehicles there are to add. Sure a subtree might look neat condensed in a single line of a tree, but then you only have a fraction of its vehicles represented, and lose out on space for the main nations vehicles.
Take for example Britain, now try to tell me how you will reasonably represent the Indian nation there with current subtrees? Issues like this are what is to be fixed with this suggestion.
So you’re telling me that you can select foldered vehicles, without issue, you can Switch between nations, and you can even make compined lineups of Aviation and Army without it being clutter, yet this is too much?
I genuinely don’t see how this is so much of an issue. Maybe if you can elaborate on which exact part of this is introducing so much clutter it can be improved on, but to me this is only concepts that are already present in game combined to improve the implementation of subtrees.
If you have any constructive criticism, please feel free to share it so the suggestion can be improved. In fact large parts of it have already been improved based on other players suggestions here, so more is always welcome.