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.
Like many previous ETFs topics, people always question ETFs, some think it will have a negative impact on Bitcoin and don't like the appearance of ETFs...but the problem is we can't do anything to prevent it. So it really is a waste of time when we spend time talking about things we cannot change. And in my opinion, the existence of Altcoins will have a positive and negative impact on bitcoin, and what we need to do is adapt to it because we won't be able to do anything to change that even when we don't like it.