You have two questions here. Do we really need BTC?

Well, if you have the wealth and you don't want its value to keep draining then BTC is your hedge to it and you can also make it grow with BTC. This is just one purpose. There are more reason why you need BTC, one is decentralization.

Is it alright to earn profit with bitcoin coming from the new investors of bitcoin?

Its in the market, when people bid they allow themselves to be vulnerable also to losing money. If early investors profit from the new ones than that because they are earlier than you