Considering City has a lot of achievements and a lot of money as well, unless they hire some terrible manager, they should be fine after Pep leaves. I am not saying that they will be this good, because they have done what people thought was impossible, so it may not be easy and they may fail, but I think it's obvious that we are going to see them still do fine, maybe even win.
Manchester City has very strong finances so they are able to build their squad to be better every season, with strong financial support and a qualified coach make Manchester City is quite perfect as a team at the moment, with strong finances it shouldn't reduce performance from Manchester City If Guardiola leaves in the future, Guardiola future is currently uncertain considering that Guardiola is not yet willing to extend his contract at Manchester City at this time, looking at Guardiola current situation, it is very likely that we will see that next season will be last season for Guardiola at Manchester City.
Pep to Spain would be perfect though, players there knows how to pass, and that tiki-taka system would be quite good, I think it would be very successful to get him and let him do it. I also believed that if Barcelona got him back, he is one of the few names that can get them out of the debt and trouble they are in, but I doubt that will happen anytime soon.
Guardiola is one of the coaches who has been the forerunner to the development of tiki taka in Barcelona so far, so when he coaches the Spanish national team, we will most likely see significant changes in the Spanish game in the future, but I personally think that other possibilities will occur in the future, Guardiola contract will end soon in manchester city, as a coach who has won the championship trophy with 3 clubs in 3 different leagues, at this time of course there is a big possibility that Guardiola might want to create a new record for coaching two other teams in the Serie A and Ligue 1 later, so that he will be known as the most successful coach in the top 5 European leagues in the future.