I get it. You need a steady stream of income to keep the game running. You deserve to make money ... I *want* you to make money, so I can keep playing the game. But the way you're making cash now, introducing increasingly powerful premium tanks, is harmful. It makes all my existing tanks less fun and less viable. And you can't keep this up; you can't keep making each new premium a little more powerful than the last, because that warps the entire game balance.
Here's a better idea. Instead of selling premiums, you should only LEASE them. Players pay cash (or gold) for the right to drive a premium tank for a limited period of time. As a straw proposal, let's say that renting a Loewe costs 50 cents a day, $5 a month, or $25 a year. When the lease expires the tank disappears and I have to pay again if I want to keep driving it.
From your perspective, WG, that means you get a reliable stream of income instead of a one-time purchase. You don't need to keep developing new premium tanks to make new income, so you don't have to keep digging for goofy prototypes and spend time and money creating and balancing them. You don't need to keep escalating their power level to convince people to lay out money year after year: we'll keep paying to rent the same premiums we've always had. And you have more freedom to rebalance those tanks, because the players aren't stuck forever with a bad tank if you nerf it; we just let the lease expire, and then try a different tank.
From the player's perspective, it means spending money on a premium tank is less risky. I can try it for a few days or a month inexpensively and then only renew the lease if I like the tank. And if it gets nerfed or becomes obsolete, it's not as great a loss because it wasn't a permanent commitment.
I realize there's no way to implement this now. You've already sold tanks permanently and players would be outraged if you said, "They're gone now. Want to rent them back?" Instead, you're going to get the same effect by introducing a stream of increasingly powerful premiums that people buy, but stop playing after a couple of years when you release a newer, more powerful premium. To you that's just as good as a lease, plus it makes us buy extra garage slots.