Saying that a good side effect of piracy and devaluing music is new artists dying out (because “only their first couple albums are any good”

is absolutely and utterly ridiculous.
Whoops, I definitely didn't mean to say that. I forgot to excise the "piracy/devaluing music" part when I quoted you; I don't agree with that part. I simply meant to agree with you that
declining industry revenue would be likely to lead to shorter average careers; I just don't believe that piracy or "devaluing music" have been the major contributors to that revenue decline (unbundling of the album and greater choice in where to spend entertainment dollars are the major contributors IMO).
And then I'm not saying that those shorter careers are a "good" side-effect of declining revenue, just that a universe with fewer career bands would still be a universe I could continue to live in happily.
Passion doesn't necessarily decrease as an artist ages.
Oh, of course not in all cases, that's just the general trend; I agree that there are plenty of exceptions.
You picked an interesting triumvirate with Dark Tranquillity, Amorphis, and Paradise Lost though. All three of those bands started their careers with 4-to-6 albums showing almost-unprecedented evolution from album to album. And though they generally went from "more extreme" to "less extreme", it was pretty clear that they were naturally evolving towards where their passions were taking them, not because they were "selling out".
But then all 3 of them said, "hmm, whoa boys, it looks like we've taken it a little
too far for our fans to follow, and now that we've got kids and stuff, let's dial it back to where we were a few albums ago and just stick with that style that's safe and we know how to do well."
No doubt Dark Tranquillity and Amorphis (I have less personal experience with PL) are making very "high quality" albums that benefit from their long experience. It's nice that they exist, and I'll occasionally listen to some of them (I particularly like Amorphis's first "safe" album, 'Eclipse').
But if forced to choose between yet another "high quality" set of songs expertly repeating the style of 'Eclipse' or 'Damage Done', or a new band creating something
as unique and unheard-of as 'Tales From the Thousand Lakes' or 'The Gallery', I would choose the latter every time.
But I admit that I might be biased somewhat more towards the novel than the average person.