Why do you think good altcoins fail to keep going? It seems there are some that are good and people collectively have put in tens of thousands of hours supporting, but the team decides to give up midway.
Do you have any examples in mind?
I would guess that most are unique when released, then slowly become overwhelmed by clones and then newer faster, better alts. Alts are basically a big pump and dump scheme anyway. That isn't to say that some of them aren't good projects, but one people are done pumping them and dumping them for profit, the interest in them slowly fades away.
I think Earthcoin looks good. They set up a huge network at one time, as can be seen in the merchants section of their website.
You can still mine it and there is a decent marketcap still for it.