Solely on the notion that Disney had managed to keep characters like Mickey, Minnie, Donald, Goofy, Pluto, et al. alive for years after Walt's passing; Jim figured if anything were to happen to him, Disney would do the same for the Muppets.
But, naturally, things would be different if he didn't die, but not everybody believes Jim would have continued to be involved with the Muppets to this day if he was still alive, considering he was in the process of branching out into a full-fledged movie production company at the time of his death (as well as turning the Creature Shop into a company that would supply animatronics and complicated puppets and suits for other Hollywood movies, much like George Lucas had his visual effects company Industrial Lights and Magic). Frank Oz has even predicted that Jim, more than likely, would have developed in deep interest in CGI animation, and may have gone on to be a key figure at Pixar.