Chasing Manchester United premier league win will be very difficult or impossible to achieve, atleast in this our time. 6 English Premier League trophies means 6 seasons and looking at it, the highest Manchester City can go before being dethroned as the indisputable champions of England is 1 or 2 seasons. If you look very well, Newcastle is just building and Manchester United is gradually coming back to form and Liverpool is there to always contest. There will be a time the reign of Manchester City will also come to an end.
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
However, I believe that when Manchester City fail to win the title in a few seasons and or experience a drop in form then of course Pep Guardiola will leave. Indeed, at this moment or maybe in the next one to two seasons Manchester City can still compete well in the title race. But yes, because Newcastle and Manchester United are already owned by wealthy businessmen, of course, these two teams will also be able to get better in the future. Therefore, I believe that Manchester City will not be at the top forever and it will not be forever either, Pep Guardiola is at Manchester City.