I think this is just to serve as a means where people will easily trade their coins and convert to fiat at anytime they like. This is really one of the things that crypto need to bring more liquidity to the tradeable coins and the entire market.
That's right. If we didn't have centralized platforms emerge, how would people else get into Bitcoin? The more of these platforms emerge, the better it is because more competition in the space leads to better and cheaper services.
From there people can always have their coins be sent to the wallet they own the keys of. There are so many solid mobile wallet clients available that you don't have any excuse to point at for not taking care of your own keys.
Another factor that will continue to stimulate people to not store their coins on exchanges and online wallet services, is how they keep getting hacked. The only way to avoid losing coins is to avoid using these centralized platforms as a wallet.
Without the centralized crypto economy growing it would stay a niche for ever. Don't forget that.