Bitcoin doesn't care how much of it you have, as it uses Proof-of-Work not Proof-of-Stake. The only thing that matters is you computational power. Now it is true that large banks could just buy lots of ASICs and try to beat the network, but that would mean a continued investment: As soon as they stop mining, the network would return to normal.
They can hurt bitcoin (not that cheap anymore) by making it unusable for a period of time, but unless they are willing to keep mining until everybody else stops, they won't be able to kill it.
As far as usability goes i don't think bitcoin is that hard: Download an app, scan a QR-code, send money. Not that complicated. The transaction rate is a problem, though.
I agree that some people are too enthusiastic about crypto, but I think it still has its place. It doesn't have to kill banks.