I think that's ridiculous to say given the fact that rock fans and pop fans come and go as well. If anything, outside of country, hip hop has allowed artists the most longevity in recent years. Second, it would be on Kendrick because if your last album did not garner their interest, why would they be excited about what you do three years from now if you basically disappear? There are other artists that are very talented. He's not some legend where he can disappear for that long and just drop and still be the top guy. You're acting like historically artists were not putting out music constantly. The general music audience may have a short attention span, but hip hop is no worse than any other genre besides country which operates in an entirely different world.
Rock and pop fans aren't demanding Taylor Swift mixtapes. Rock fans aren't saying "that AC/DC was aight, but put something else out" Kendrick puts out a highly acclaimed album, best first week numbers ever for him, still selling well, and we don't appreciate shyt and want him to put more shyt out.
We can't complain about the quality of music and then think classics get pumped out in months. Sometimes you need the artist to live life and actually have material to talk about. That's what Kendrick does. If it only takes a year or two to forget about you, then yes, we have really short attention spans. Did R&B replace D'Angelo? No. Did R&B replace Maxwell (and let's be real Maxwell was putting out crap before he went away)? No. Even pop acts will be given time to craft stuff if they were poppin before.
If we need artists to put music out at a Curren$y-like pace, we're never gonna be able to appreciate timeless shyt when we hear it.