I'm incredulous about it being easy/cheap to get 10% of a stake of a well functioning coin. If you can get 10% of a stake without buying and want to profit from it, the easiest way is not to give back the 10%, and sell the coins on the market.
In PoS economy it is very risky to buy a big bank or exchange business from its previous owner. Think about a situation where the owner of a bank/exchange with big stash of customer coins in their possession has decided that he is more a risktaker entrepenour type of person instead of a person that runs an established mature business and he wants to cash out and start over from scratch and take new risks on some other competing emerging new kind of coin. Quite natural event that I guarantee is going to happen thousands of times.
You understand that long range attacks have proven impossible in simulations. So if a bank buys a large chunk of coins and waits the required number of confirmations, then the previous owner cannot launch any attacks. Or am I misunderstanding your premise?