Based on how the average head coach/manager cycle goes, it's usually between three and five seasons before they move on or get sacked. OGS even in a hypothetical position where he is a first team head coach and has a football directorship hierarchy working with him on all the facets of Utd, will still have a lot to do. Guardiola famously ran himself into the ground at Barcelona and just said 'feck it, i need a break even from my favourite team'
God only knows if OGS can create a trophy-winning team, but awesome if he does but we've seen many good managers do that and then they fail when their old team breaks up and they have to create a new one. That's why Sir Alex is so reverred because he did it 4-5 times for 1 club, and there really aren't many managers out there who can boast that. He might be the only one who's done that at this level so there's an extremely small chance that OGS could do it too.
But if he can do it and he doesn't significantly drop the standards like SAF did then let him continue, but in the more likelihood that OGS could hypothetically make one great team and fail to create a new one to emulate or even surpass it, meaning in the most cases a club will be looking to part ways or sack them, then we look to try to keep hold of him and move him into a football director position where he can still have an impact on the club and doesn't have to do the day-to-day stuff if there's a scenario that he's burnt out or has taken the team as far as it can go.
It's like how Matthias Sammer's coaching career has gone where he was a successful manager for a short time with Dortmund, then became a sporting director with Bayern after the Dortmund gig declined, but was able to be a part of a Bayern Treble team.
Tl;dr version - OGS could be successful but as current cycles go where 3-5 years is a typical cycle before a decline hits (unless they're some SAF 2nd coming) but if/when a decline hits, we shouldnt get rid of OGS and we should try to keep him on as a football director due to his vast knowledge of the club. Alternatively if he runs himself into the ground Guardiola at Barcelona style after success, then we should try to keep on as a football director and he has a less hectic role and more knowledge of how the club is run if that were to happen.