If you get stuck in an ivory tower then you’re kind of lost, and that’s what happens with some of the older bands; they sort of lose touch with what actually is reality. – Phil Cullen, Def Leppard
With a few exceptions like BEHERIT and ASPHYX, the nearly-universal rule is that metal (and other popular genres like rock, techno, etc) artists/bands get worse over time.
Usually, there’s an inspired but semi-incompetent first album.
Then an ambitious but half-complete second album.
Finally, the dreaded third album, which either makes the band or breaks them. They’re at their technical peak but losing vision.
After that, they’re re-interpreting their influences, being influenced by other bands they’ve toured with, etc.
They don’t seem to ever get it back.
Why is this?
Much like modern society itself, the music industry is a process of detachment. You detach into thinking about sales figures, being on tour all the time, trying to manage your money, wasting time with women and drugs/alcohol, etc.
Eventually, you’re living in a new place, practicing less like a passion and more like a job, and thinking about how to be popular.
Whatever inspiration you had is gone.