Marr can reliably go on tour as the main act, he can go on tour as the support act, he can be a special guest, he can join different bands, he can work on movie soundtracks, he can create books, signature guitars and do speaking engagements.
All without any drama, he has a nice, super healthy and comfortable life and a happy, stable family etc. And 99.9% of the population have no issues with the guy
He would have to be really bored of all that to go anywhere near and attach his name to Morrissey and the potential car crash of cancelled dates and political pr disasters, for some bloke he doesn't like and a bloke who doesn't like him much either.
Marr doesn't have to be Einstein or a perfect angel/the nicest guy in the music industry(whatever the weird standard you hold him to for some reason)to want nothing to do with that shit in his 60s