I see there are 2 sides to this argument so I thought I'd make a summary of the pros and cons.
+ Greatest player in history plays for Man City.
+ Scores 30 goals and gets 30 assists a season.
+ Plays the best football you're ever going to see.
+ Tends to make everyone around him play better.
+ Will probably make Pep stop doubting the side.
+ Brings in a ton of money.
+ Upsets millions of rival fans.
- People might be mean about us buying him.
- We don't know how much of someone's else's money it will cost.
Rarely played a decent game with Aguero.
Plays in a free role, with team built around him.
KDB often plays a free role in our team.
Will be playing in a higher intensity league, at 34 where many of our younger attackers have picked up injuries.
Takes all the pressure off Pep , to build a new team & allows him to do a Pellegrini, seeing out a 3 year deal tied to Messi, then leave, with whatever situation behind him.
Teams will be more keen to stop us, again rather than stopping Liverpool, whose turn it should be for the treatment.
Players who don't fit in with Messi, will effectively be finished at City, even if they are key members of the 'team' with all the possible resentment that brings in, with their team mates.
Pep might go mental & leave, if results don't go his way & he loses inspiration (we do still have to defend & Barca can't do it for example) .
Imo Messi will have no problem in the Prem but no guarantee the team will not lose out in other areas.