The game is designed in a way that it forces you to buy it or you lose. Since credits are rare, and gold to credits conversion is a joke, you will have to end up spending the money. Also higher you go in a tree = higher repair cost = also more gold ammo usage.
On every new battle you lose a lot of money and they nerfed any type of moneymaking (tier 4-5), so they can sell you those *premium* tanks.
Need money --) Make a OP premium tank --) No1 buys it anymore --) Nerf it --) Design a new premium tank --) Sell it --) Repeat
A simple math and you will notice that its simply not worth bothering any more with this game.