that is a problem but i don't see it being a big one. and the problem is not what you think, the problem is that we need more exchanges so that the volume distributes more between them and as a result we see less power for them in controlling the "price".
the good news is that it is already happening. in early years there was only one big exchange and some small one s (Mt Gox) it was controlling the entire market then other exchanges grew bigger and nowadays there around 10 big exchanges i believe. that shows growth. if we keep this track in a couple of years there will be 20-50 exchanges all around the world with volume being distributed among them.
2. If top 50 people hold 50% of BTC and BTC becomes the global currency, we may face once again the previous era. Being slaver to certain people.
they don't control bitcoin by having (temporarily) a big percentage of the total supply among multiple unlinked exchanges.
bitcoin is PoW not PoS.