Yeah I think you'd have to view this through the lens of had Sir Alex been exposed to modern tactics enough to make required adjustments. I think we saw later on in his career that he could do despite people always saying he wasn't a great tactician.
I know you get into a lot of hypotheticals when you start to consider things like that but I think it's more fun to do that sort of guess work like "what sort of player would Beckham be if he came through in 2010?"
Otherwise it really does turn into older = worse.
No debate that if you used a time machine and sent Barca of 2015 back and switched them for 99 Bayern what would happen.
Even taking context into account, there is not a single advantage you can give to United compare to City 2023.
Premier League: City got 89 points, 10 more than us. They won the league a few weeks in advance, and actually could afford to just get 1 point in the final two matches and still win the league by 5 points. United on the other hand won the league by a single point and had a must-win final match. Bear in mind, EPL is the best league in the world now, it was probably third or fourth best back then.
FA Cup: City had never to come from behind, in fact, the only goal they conceded was the penalty from Bruno in the final. United was arguably outplayed from Arsenal and kinda needed a miracle goal to go to the final. More exciting yes, but not as good as City who scored 19 and conceded 1. We scored 12 and conceded 3, in comparison.
UCL: City did not lose a single match in the entire competition. Scored 32, conceded 5. Dismantled the other two main favorites Bayern and Real, defeating them 3-0 and 4-0 at home while drawing away.
We had only two wins in the group stage (albeit faced Bayern and Barca), did well against Inter, but needed a goal in the final minute against Juve at home to draw, and then needed to come back from 2-0 when away. Then Bayern totally outplayed us in the final, and we fluked it as much as Chelsea did 13 years later. 29-16 goal difference. City won 8 and draw 5, no losses. United won 5 and draw 6, no losses.
I know that we did it in style and it is far more exciting. But if you are so much better than the others, you do not need to come from behind an absurd number of times. You just lead and dismantle the others, which is what City did. Add to that, the level of competition was higher in 2023.
I think that City 23, Barca 15 and Bayern 13 are the best and probably the order can be argued to death. But it has to be one of those three. I would put Barca 09 behind them. United and Inter at the bottom. Bayern 20 no idea, they were good but it was such a weird season.