No, such is not fraud.
Have you ever played League? You can buy champions there. Those champions can and will change drastically between patches as the developers try and balance the game so it is fun for everyone. Abilities might get entirely reworked!
In Warthunder, your vehicle will only change in mechanics and looks to better adhere to its real life counterpart (bar a few soft balancing factors around crew).
Instead, in warthunder matchmaking will change.
If premiums did not change with their matchmaking, THEN you should be asking questions because veritably, that’d be really screwed up.
Even Magic the Gathering/hearthstone will change the cards they release in their digital platforms. Only reason they don’t with physical pieces is that you cannot remotely alter cards. However, they do release new rulesets constantly to keep the game engaging and fun.
Warhammer (the wargame variant) is the same. You can dump a ton of money on your figures and then a new edition comes out that can completely change your viability. That’s a good thing.
Age of Empires 2: DE has premium nations in form of DLC. Those nations get completely reworked and updated as the meta evolves and balance requires it.
I cannot think of literally any competitive game where if you buy something, that will never change. Hell, even non-competitive games will completely change and alter what you bought like the MTX-riddled variant of Runescape will (and in OSRS players actively demand changes and nerfs, altho that is a normal sub based game rather than direct purchases). Or even Crusader Kings 2/EUIV/Stellaris will change its gameplay drastically between patches.
Star Wars empire at war, a single-multiplayer competetive RTS? You guessed it. Gameplay changed drastically by patches!
I have to ask: Do people who ask such questions play any other competetive game than Warthunder?
If this were something like AoE2 DE, people would be pitching a fit if the DLCs were NOT changed and adjusted to ensure a fair and balanced playing field where player skill matters most.