There's nothing actually restricting coins to a particular use, so people would avoid exchange fees and technical complexity and gravitate towards fewer (if not only 1-2) currencies.
If bitcoin were perfect in all aspects maybe this argument would hold some water, however I highly doubt that's the case.
I think market has already spoken on this matter, just look at the money flowing into altcoins, any blatant copycat coin can get to 10k BTC market cap overnight.
Scalability concerns and the uncertainty of proof-of-work concentration are the top two issues that are eroding bitcoin's dominant position.