I think it is not that much difficult given how Guardiola has managed City ever since he took over as manager. He has made then PL champions 5 times out of 7 seasons and that really shows the kind of spectacular record Guardiola hold rights now. I think in 8-9 seasons he can make them equal to United if things goes like this. City are in control in whichever way you see and despite United and Newcastle improving, I don't see them getting dominant the way City are right now. I would say its very much achievable if Guardiola says for long term.
If Guardiola continues to manage this team for a long time, he can do as you say. But he has been managing for 7 years, how many more years will he want to continue in this team? I think after a while he might get bored and want to move to a different team or retire.
Guardiola's influence on the team is huge, the reason the team is at its current level is not just because it has good players. Pep's effort to hold the team together and his tactical success is too great to ignore