your question is like asking if the market was filled with more apples would people stop buying oranges and only consume apples?!

as far as usage as a currency goes, none of the altcoins have that so far. if any of them succeed in that task then it has nothing to do with their cap. people will go there and use it.

as far as investment goes, the altcoins with no cap or huge number of coins will never have a decent price and will always have a declining one because of the bigger supply than the demand. so they become worthless as an investment. in other words people will stick to bitcoin for investment.