I have two question-
1. Exchanges are being the giant although BTC holding by them are from customers. Still it's possible that they are trying to be a controller of the crypto community which is very bad I guess.
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.
Think at from the point of macro economy. In my opinion, it's something we need to talk.
Like you said, it could be owned by exchanges, but that does not mean that it is the exchanges that own everything, there are still going to be people’s bitcoin on them, and remember that there are many traders using some of these exchanges, which they have to keep the money in the exchange wallet and I believe that this would be part of the reason why we have limited number of people holding majority of the coin.
This could actually be a problem anyway, if the holders of these coins are individuals, it could still cause inflation, but I doubt if individual would actually be the one holding more of those cryptocurrencies, and don’t forget that the only way it can work against us is when government makes it the only currency of the world, otherwise, people would shift attention to altcoins making those bitcoin holder to lose.