if they want they just can buy all top10 coins and rule the market
It's not that easy. If we look at how splintered the market in its entirety is, one entity or a group of entities will need to
extremely overpay for their coins in order to own even a semi large chunk of coins in the end. Another thing is that banks or other wealthy entities don't trust exchanges and will not participate in them; their only way is to accumulate coins through private deals, which is getting increasingly harder with so much OTC demand.
Miners have plenty of buyers to sell their coins to, mostly at 25-50% market premiums due to their virginity. Services connecting OTC traders have to deal with far more demand (ie, more buyers than liquidity providers). People holding Bitcoin don't really want to sell many coins anymore with how the price will be going up. It's not easy to obtain Bitcoin anymore, especially not in extremely large numbers.
I think banks will continue to function.No cryptocurrencies can challenge or overtake banks.People all over the world have faith in banks and use them.Banks are save and secure unlike cryptocurrencies which are very volatile and risky.Although cryptocurrencies are profitable but still we cannot believe them.One can also get good interest rates from banks too.Banks are functional from traditional time period.But these cryptocurrencies have started recently.Nations are also not supporting cryotocurrencies as they are decentralised.