The only guy who could've taken over for Miyazaki as the natural successor, Yoshifumi Kondō was literally worked to death. And they never found anyone else unfortunately. Nippon TV has at least been with Ghibli since the 80s so that's better than some new company with zero relationship to them taking over I guess.
I also wonder if Miyazaki not retiring is less so him making another film and more so writing a ton of scripts and sketching out concepts for future Ghibli films post passing,