Keep Tanks After Unlocking Next Tier?

Lert: If you like a tank, keep it. If you don't, don't.   Depending on your skill and whether or not you have premium, you can start losing credits for bettles at tier 6 ~ 8. Only the absolute best players make money reliably at tier 9, and even they don't make enough to make that their sole source of income to fund a tier 10.   Besides, if you ever want to platoon with other players or take part in WG's many tournaments and events you're going to need a spread of tanks anyways.