Here’s the point, he probably should not be replaced as-is. His trademark stubbornness has gotten the free software community in trouble before, and while admittedly that same stubbornness is what has allowed the FSF to persist in the face of corporate attacks over the years, that same stubbornness has also prevented the FSF from having a firmer standing in the software community, due to its own ideological purism and reluctance to collaborate with less strict actors like the Open Source Foundation. During the time where Stallman was temporarily banned from the FSF, I could see an ideological move towards leniency. Before Stallman left, they kept complaining about users that didn’t quit the entirety of proprietary software cold-turkey (and socially isolated themselves as a direct result). After Stallman left, though, they started to go for an approach they call the “freedom ladder”, where they request people to start using as much free software as they viably can.
But if he absolutely has to be replaced as-is, it’s incredibly difficult to find somebody with the same degree of insistence. Eben Moglen was, in my opinion, the most viable candidate, but sadly he was recently outed as an abusive employer.