Agree with much of this.... "longevity" is an overrated quality in rock/pop...witness the hoarse, prehistoric cadaver-driven cash-cow monster that is today's Rolling Stones... What the fabs have achieved is so much more impressive... i.e. longevity of appeal for a (relatively) brief career.
The Beatles undoubtedly split at the right moment in my opinion.
Nobody is saying that longevity in and of itself is worthy of commend. You are going for the worst case scenario, but even the Stones released some of their best stuff in the 70's, according to the majority of their fans. The Beatles split at the right moment because they couldn't function as a band anymore and smartly realized that they could never put together another album with every member's full involvement in it. It was the best decision for them. But that doesn't mean that a band/artist can't release great albums over multiple decades. It also doesn't mean that those bands/artists are not worthy of admiration for doing so. Scott Walker released 'Tilt', the album that is now widely considered his best and most original 28 years later after his (also acclaimed) debut.