I think OP is correct and I would merit the post if they participated actively in the discussion ...

I think there are several effects in play.
First, there is the tokenomics problem. Many altcoins are massively premined. And in some moment the original holders (founders, ecosystem, ICO buyers, seed investors) will dump their coins. Often a coin has a lifespan of a couple of years, and then once the OG dump begins massively, they simply fade away.
But there is also the novelty exhaustion problem. Altcoins often try to find investors telling them that they are something "new", they solve a new problem, or they try a new algorithm. This is a problem because there are only very few novelties of this type that can't be adopted by other altcoins. Eventually they will need to create some kind of sustainable "business model", and most fail at that.
Thus the effect of many older altcoins "slowly dying" is very easy to expĺain

How to find gems? Well, I think old coins without premine may have the highest chances, but besides a few (like XMR) they currently are not talked about much and have very low market cap and volume. If a community manages to create hype of such an old coin again, that could be a massive opportunity. The second group I'd look at are those with a premine but which was already a very long time ago. Ethereum is the prime example, but there are also some smaller coins like Tron created pre-2020 where the "founders dump" effect may have already happened and they have created an active ecosystem.