I'm not sure if there's any point in having such discussion as we not going to develop an alternative universe in which altcoins don't exist.
Bitcoin is here to stay and so are altcoins. There's not much anyone can do about it, other than maybe governments banning them.
That being said, you can't ignore the positive effects of having multiple cryptocurrencies in existence on Bitcoin. First of all we have huge crypto exchange market, and altcoins act as a testing ground for many solutions/features that (if successful) could be implemented in Bitcoin.
This begs the question, what if we had only Bitcoin? Wouldn't the level of adoption outgrow what it currently is at the moment?
Good question. If everyone could only use Bitcoin, I think the network would either get completely clogged or it would have to evolve into something completely different. In the first scenario, Bitcoin would be deemed as a failure in terms of transferring value and could end up with less demand than it currently has.