There are two things wrong here and people only responded to one of them so far.
The obvious one that when we talk about adoption we aren't talking about a situation when you can buy anything anywhere at any time. That would be more like replacing fiat which is not the goal.

The other one that nobody addressed is that altcoins will never become mainstream or even be used as long as their purpose is to be pumped and dumped. For years the altcoin creators only made new projects to make money themselves, hence the premines and ICOs and PoS and so on. Until that changes, we won't see "other cryptos" ever be used for anything meaningful.