To be honest, I barely invest in altcoins for the long-term because I don't trust them much based on the incidents we have seen in the history of altcoins. LUNA is one of the biggest examples of the altcoin disasters we've witnessed and thankfully I wasn't a part of that project, but it haunts me until this day and such things are the reason why I generally stay away from altcoins when I'm thinking of making long-term purchases.
However, if you ask and even if I have to answer hypothetically, I would say that it's probably the use case of an altcoin that I would be more interested in because that is the only thing which might give you an idea if an altcoin is going to survive for long or not. Look at ETH, for example, it brought a new wave in the industry, allowing developers to create and use their own tokens and coins through smart contracts, and that is still in use, which is the reason why ETH is where it is today.