Habit. It's like to you this is something he'd just stop doing, lol. He's old, he'd do it anyway, because it's what he did for nearly 7 years now.
You also work in terms of the players City have. Why? Think what would happen if Guardiola came back here. Who would he get rid of? Suarez is your Aguero, so he'd stay unless he wants to go. Wingers would stay. Coutinho would probably go. Busquets would be kept as a Fernandinho. Both fullbacks who are rather average as defenders would likely remain. De Jong comes in for Rakitić. Pique would probably stay for the lack of viable CB replacements and would rotate with Lenglet.
And with this kind of squad, there are countless defensive issues, many amplified by the Messi having to play as false #9 forcing Dembele & your LW narrower and putting Semedo and Alba under pressure. No doubt it's a better system than the one you currently have, but Messi would continue to do what he did for years. It doesn't just stop on Guardiola's "says so".
Habit? Pep is a master at breaking the bad habits and implementing his own style. At every team he went he implemented his style.
If Pep would came at Barcelona?
Rakitic out, Arthur and De Jong starters from day 1. Tailor made for his football. Busi could up his game a bit considering we'll be playing the style he is used to the most, even if he wouldn't be able to match his 2011 form obviously.
Suarez could stay, but he is nowhere near guaranteed to start like he is now. Only Messi would be guaranteed to start probably.
Pique is not a problem. Lenglet, De Ligt if he comes. Umtiti sold probably, Todibo promoted. Dembele starter and would improve probably at a much faster rate (similar with Sterling who was made fun of on redcafe in the year before Pep came, and is now top 5 players in the whole league arguably).
Coutinho is an unknown for me. Could still be sold, or could actually be tried more in midfield as an AM, which is something we've asked a lot here, but to no effect (basically played 90% as a winger no matter his performances there, which sucked usually).
Alba would be great under Pep. No doubt. Semedo would improve as well, even you said he is a good player.
Basically if Pep came back he'd improve our 3 most problematic things. Position and role of Messi, would get rid of the veterans running things since he will be the boss from day one, and would improve the stagnating talented players we have.