To be honest, there's no point even thinking of a top manager first approach like Tuchel (example) until both the seniors and Messi is gone too.
Messi is putting the numbers where it doesn't make sense to replace him, and we don't have the money to either, but his influence is too much for tactics to overrule him in matches.
Then there is his influences on the club transfers. How many years have we spent chasing Neymars return, or Aguero before that, or De Maria before that.
A competitive team does not mean profiles to make the team better, it means individuals Messi likes that he thinks he can play well with. That is a problem.
Look at Liverpool, City, Chelsea. One thing none of these have are legacy players in the lineup. They can sign, sell and replace unbiasedly.
Meanwhile we're going to go into the 2021/2022 season with Alba, Pique, Busquets and Messi as our core players on every line in the team. One decade later they are all still here throwing BBQs or meeting the head of UEFA with our president. Think about it.
Good luck having Tuchel or Nagelsmann coming in and trying to imprint his methodology. Tuchel was already being questioned for his ideas with the big egos and fans of PSG. Imagine here. While Klopp would probably do a Camacho and leave within a month.
It is what it is.