Coins that are suitable for long-term investment are those that already have a product. By owning a product, a coin or token will continue to be traded in the market, especially if the project is able to make a profit, I think investors will continue to hunt for the token or coin and expect profits from the company.
Seen projects launched their product but token now is dead. Unfortunately there is no guarantee that a coin that solves problem will not really die, its all up to the community supporting the project. For users to support it means it really will have a good impact to them not just because of the price though its one of the factor. I wouldn't use ETH if it drops so much that disappoints me anymore. I would rather be going into ADA that way i'm way getting them for a very low price. What problems does your coin solve may not really be for a long term which is why devs discuss about scalability.
I'm so disappointed with those projects who told that they will be great when there product release and turned out nothing even though they released their product that's why as of now I rather go with stable coins if I want to store safely and without worrying on volatility or just go with the top coins for profitibility. And discussing with devs on what will happen in future is useless since for sure no one of them will tell bad or struggling words since they don't want there supporters get upset on them.