Don't think it takes three years to rebuild this side. Two tops.
Pep built his own team from the ground up in about six months in his first season, it just took a full season with it on the pitch for it to fully bear fruit. From February 1st, 2017 onwards (six months after Pep's game) we lost two league games in an entire calendar year and eventually won the league with 100 points. And that was starting from Pellegrini's last season of 4th place, 66 points, and our most fragile squad post-ADUG takeover.
By the time Pellegrini left City his strongest line-up was:
Hart
Sagna, Kompany, Otamendi, Clichy
Fernandinho, Fernando
Navas, De Bruyne, D. Silva
Aguero
The team Pep won (most of the) 18 games on the trot with was:
Ederson
Walker, Stones, Otamendi, Delph
De Bruyne, Fernandinho, D. Silva
Sterling, Aguero, Sane
In the space of 15 months Pep only changed five players to the starting XI but it was all he needed to shift the mentality and success of the entire club. Despite what people say about the players Pellegrini left for Pep (on paper), the situation was bloody dire and we all remember what that 15/16 season was like. What Pep's currently got to work with is far, far superior to the team he was given when Pellegrini finally let go in the summer of 2016.
There's no reason why, on the opening day of next season, our starting line-up can't be roughly similar but vastly improved in terms of productivity and efficiency. Fuck, even with the current crop of players available, if we get any of the key names back that we're missing then the situation immediately improves. Rebuilds aren't about changing all eleven players in your strongest team and it won't take Pep three years to do it.
Ederson
[solid new RB], Akanji, Dias, Gvardiol
Rodri
[creative new AM], Foden
Savinho, Haaland, [exciting new LW]
Two years would be more than enough for Pep to completely change the future of this club for after he's gone, but even renewing for only one year would definitely allow him ample opportunity to set this team up so that his successor can keep the good years coming. We'll definitely drop a level when Pep goes but there's no reason why he can't put us in a position to replicate what we managed between 2011 and 2014 - and he'll only need an extra year to do it imo.