but at current size of bankroll it´s just impossible to gain enough to make this worthwhile.
therefore all the profit goes to the same 20-30 people wo have invested 50+ BTC or more.
i mean in order to have only 1/100 of the bankroll you already have to invest 72 Bitcoin.
I don't see any way around your problem.
If they put a hard cap on the total bankroll, say 10k BTC, then people will start trading accounts. And guess who will end up with the accounts when their price gets bid up? It will be those same 20-30 people with the most coins.
You're always going to end up with the people who want it most being invested, whether you limit the bankroll size or not.
When you don't limit it, people get diluted to the point that it's not worth staying in for the people who aren't really dedicated to it, and if you do limit it then the price of getting in goes up to the point that it's not worth getting in for the people who aren't really dedicated to it.
The only difference is that when accounts are traded, the site owners don't to make any coins, whereas when it isn't they get to profit more by the increased publicity and volume that a bigger bankroll can attract.