I think Dombrowski made and makes that decision as much as Middleton, and it's not supposed to happen until prior to the 2028 season (and even then would be contingent on Dombrowski retiring. Different story if he's fired). Fuld hasn't even come back to the team yet (he is still listed as VP and general manager, albeit on the biz side, and doesn't graduate for a few more weeks) and with Mattingly it was just to prevent another club from poaching him as GM (it's unlikely another team would make him PBO, due to both of his limited experience - only one year to this point - and the fact that very little high-level stuff is delegated to him under DD).
To me, if DD has failed badly enough to be pushed out, I don't know that his right-hand man should be the successor.
Fuld's future role is more interesting to me, tbh. As CEO Middleton has also been de facto filling the role of President since Gillick (who had temporarily taken over from Monty, who took over from Giles). Will Fuld merely replace Dave Buck (who retired last year) on the biz side or will he actually become Middleton's proxy control person?