ethereum is a good altcoins but i don't see them transparent in development, even the rumored eth 2.0 will launch, not even launch as of today. In my opinion, the criteria as a good altcoin are having a large community, high security and adoption.
actually there's a lof of altcoins with large community in the space but sad to say despite, most of them are still not worth it because they always end up very stagnant after all. So if we were talking about long term then for me its no other than ethereum as its very reliable ,trusted and very secured when it comes investment.
not all altcoin have bad future for long term ,some of them really need time to developt product meanwhile their investors and community haven't patience to wait it come. alot new project have good future but it dumped by their own investors meanwhile dev team serious build it. maybe ethereum investors didnt patience and sold it early we will not see current price.