I doubt that Messi can really help Manchester City. He is 33 and English football is different from the one he used to know in Spain. The game system would have to be adapted to him, which means that ManCity would have to change its current system. That would be bad for a well-rehearsed team. Besides, it would take at least a season to make it work. By then Messi will be 34, almost 35, and none of this will work.
There are some successful Spanish or La Liga players in Premier League: David Silva, Kun Aguero, Jesus Navas, Santi Carzola so I think the genius player like Messi can play well in Premier League. You made a point here that Manchester City need Messi for the Champions League, not Premier League and they only need him for their first Champions League trophy. Second or third trophy is a too far scenario to think of.
I don't want Messi leaving Barcelona on a personal note/ground but I don't agree that Messi won't adapt to Manchester city. He had played under pep the in Barcelona and the football is not far off from Manchester city style. Ronaldo went to juventus and still playing good football -not at the peak as RONALDO- age but his game is good. Messi would still play good football even if he is to play on the moon, -he has nothing to prove -
No one want to see the transfer (Messi probably did not want too) but it is life and it will very probably happen. If Messi decides to leave Barcelona, and if the transfer happens eventually I can bet with you the first club in Messi's mind is Manchester City where Pep is currently managing the squad.