Magnus not participating in FIDE events seems to have absolutely nothing to do with his longevity, it just means that FIDE is no longer meaningfully hosting THE world championship because they failed to attract the talent.
To be clear, I don't mean to take issue with the competitions themselves.
Elite sporting events are absolutely obliged to chase talent, just like any other business is. If they don't, they quickly stop being the elite sporting event. There's a reason why athletes are paid so well...