If Pep Guardiola wants to leave Manchester city his next destination will be PSG since he loves going to clubs that have the money and are willing to spend. Pep Guardiola with PSG will win them the champions league very easily. With just few signings, Pep Guardiola can make PSG into a team like Manchester city but Pep Guardiola won't be leaving city just yet. He'll try to win more trophies with Manchester city and defend their champion league trophy. If Pep Guardiola was to leave Manchester city at this very moment, everything they have built in the past years will crumble.
Well, I don't think Manchester City's results will collapse so much after Pep Guardiola's departure. The club may well win both by inertia and on the basis of what Guardiola has put into the club 
Although, of course, Guardiola's own contribution to the success of the club is decisive and one should be too optimistic, arguing that any coach would have achieved such success with such investments. The English Premier League is very competitive and you need to have something else to achieve success here.