is it in fact unfair to criticize a formerly great artist for his latter day sins, is it better to burn out or fade away
It is fair to criticize bad music, regardless of whatever the artist released previously
In terms of legacy, it is better to burn out. Imagine if Neil Young died in the mid 70s. His legacy would be very different today if he hadn't released all of those questionable albums from 1976-present. On the contrary, imagine if after his solo album, Mark Hollis just kept releasing shit until the quality was mediocre and nobody gave a shit about him anymore. His legacy would be worse than it is right now.
People who obsess over an artist's "legacy" don't actually about the music itself, but rather the mystique surrounding the performers and composers. Music artists aren't really seen as human beings who happen to have certain talents to these folks. Instead, they view them in the same way a child would view a character in a fairy tale, as a far off fable about what it means to be a great individual. Depressed musicians such as Nick Drake and Ian Curtis tend to receive the most romanticization likely due to many of these sort of music fans being depressed themselves, instilling into many the mindset that it's better to die after your best record than to live on to record a couple of mediocre albums. Again, they don't care about the people themselves, as long as their entire body of work is supposedly perfect. This mindset also implicitly discourages stylistic exploration and sonic experimentation in the songwriting process, as the possibility of "failure" on one record as the result of such attempts to expand sonically will in turn result in talk of the artist having "ruined" their discography. In reality, such generalization of an artist's body of work is just foolish because one lackluster record simply doesn't effect the quality of a good one, and treating the artists themselves as ultimately being anything other people than people with talent in their craft is fucking childish.
In short, why the fuck do you even care?
AFI went from being one of my favourite bands before Decemberunderground (they declined before that as well) to something I don't listen to new releases of with that album being the catalyst. I still listen to their old stuff sometimes though when the nostalgia hits me.