He's 82. It probably IS his last director work. He might still write or board another film or two, but the full on 3-5 year day to day process is asking a lot.
That worked out pretty well for Whisper of the Heart, Arriety, and Poppy Hill. WHisper being my favorite Ghibli film, and the other two not being pinnacle works but still feeling like they belonged.
If Ghibli keeps going after this, I imagine they'll probably either push Yonebayashi to a lead role (Arriety, Marnie) or admit they made a mistake when they pushed out Hosoda and try to get a few outside directors like him or Shinkai to do a film.
Or they just let the company fold with Miyazaki and Takahata.
Both are valid courses, but there's certainly other directors capable of making things now, and if you have the staff to do the visuals and backgrounds then you have the staff.
They tried to grow some internal talent along the way but Kondo died young, and Morita (cat Returns) hasn't directed anything since that film though he's co directed or animated on a ton of non-ghibli stuff. They can't keep letting Goro make films, he just tarnishes the legacy, Earthsea was bad and Earwig Witch was easily the worst Ghibli film. Maybe if it hadn't been CG?