I think it is impossible to have one cryptocurrency. We do not have one currency for whole world. Almost each country has its own currency. But on the other side thousands are not needed. In my opinion only TOP 50 - 100 will remain with real use.
But, bitcoin was created with the purpose of having one currency for the entire world. I agree there will be more room for having more number of coins in the future.
But, definitely the currently top 100 coins may not finding its survival in coming future as most of them are not having any unique features but serving same repeated features. When more number of coins are available with same features, I guess most of them will not finding its survival.