Well, Hendrix did do a Christmas single. Although.... it was a guitar solo! So I guess he might have done a disco single.... that was a guitar solo.
For Kurt Cobain I feel like he might have been one of few like Syd Barrett who actually would just quit. If his suicide note is to be believed, he didn't like performing much anymore. Another really cool possibility is if he had just gone completely underground... supposedly he said he wanted to put out his 4-track demos on an indie label under an alias. The only thing that troubles me about that scenario is.... I'd probably never find the alias and even if I did I wouldn't know it was him.
Of course, considering how Kurt apparently couldn't just say "no, **** off" to everybody (again reffering to his suicide note), I imagine another Nirvana album coming out in '94 or '95 with Pat Smear on some tracks, headed by somewhat less traditional, dark songs like You Know You're Right and Nobody Knows I'm New Wave. Kurt then refuses to tour and hires a bunch of symphonic musicians to make music with him. He makes an album of quirky symphonic rock headed by a song done with Michael Stipe.... .... and if everybody who says they were trying to reach him when he died was being honest, it'd also have tracks with Neil Young, Iggy Pop, and others. Then Nirvana, having never officially broken up, reconvienes to make an album with more than half the songs having been written by Dave Grohl. Dave & Krist recruit a drummer and tour the album without Kurt. Kurt drops out of the scene for a handful of years, occasionally showing up as a guest on random records, and doing gigs with mercenary musicians for a few benefit concerts. He appears live singing back-up for Hole in 1998, trying to save the failing band. He returns in 2002 with a 2-disk Leadbelly influenced, Neutral Milk Hotel-esque album he recorded on a 4-track home recorder. It gets slammed by critics as being typical, long, and uninspired. Kurt gets pissed and records a Brittney Spears cover album, but then doesn't release it. Kurt releases an album of studio jams ranging from Nirvana studio jams to him and Courtney doing a capella vocal solos in their basement, it's called "Jam It In My Butt." Kurt does a tour with Neil Young and The Pixies in 2010, his first tour in more than 15 years. Nirvana reunites in 2024 for their induction into the Hall of Fame and they record three new songs for the 4th Nirvana greatest hits album, including a cover of Eye of the Chicken by the Butthole Surfers. Nirvana then tours with The Rolling Stones' farewell tour, but then The Rolling Stones tour for another 20 years. Heheheheh.