At the time of the transfers Neymar and Mbappe were better players. Messi is the best player ever but not now. He is regarded with awe but those who argued that his decline was imminent were right. We have seen that decline, however he's still a great player and the players transfer request makes a deal feasible.
The deal is also attractive for the economic and feel good factor associated with it. It would be a mission for every City player and fan for City to win trophies for Messi. It would entirely change the perception of our club across the world. The media, Klopp etc emphasise the cost but they forget that it works for barcelona in terms of revenue generation, and they don't have international clubs like the City Football Group.