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.
That reminds me of the story of manchester united after sir alex ferguson was leaving from there. The club has been going into the disaster as it's very hard to find someone who has the same capability as sir alex ferguson in handling 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.Manchester city is relying upen guardiola at this moment. Guardiola has built a strong foundation for Manchester City, and has had a major impact in bringing in top talent. That being said, if guardiola is the main reason why manchester city able to go this far. Not even mancini was able to reach the same position as what guardiola does right now. Losing pep gonna change the club a lot. You can't expect the new coach to have same analyzation skill like him.