I think that's the point, he comes in and provides options.
You say you don't have a best starting XI but you do have some outfield players who, if fit, are in your starting XI against all but the weakest sides. Haaland, Rodri, KDB, Foden, Stones. You may rotate other places but those boys are nailed on to play against any PL team from mid table onwards. So 5 outfield players.
Last season we had to play the same starting XI a lot because of squad depth and injury to Timber but this season, bringing in extra players hopefully alleviates that. However, if we put Arsenal in the same scenario as above this season (playing any decent side) it's nailed on that Saliba, Odegaard, Rice, Saka and White start. Again that's 5, same as City. The idea is that Calafiori can replace either Gabriel at CB or play LB, Merino can slot in midfield but so can Partey, Jorginho, Nwaneri. Jurrien Timber is a superb player who can play anywhere across the back line or in midfield.
What we don't have is decent cover for Saka. City for sure have more options up front.
I get it that Arteta tends to play the same starting XI every game, and for sure Saka gets gassed in particular. Nwaneri is potentially a better player than Saka but is only 17 so not this season. But Arteta does have options this season to mix it up a bit.
We can slice it and dice it all we want but the reality is the main difference between the sides is Pep and Rodri. Aside them Arsenal are better at the back than City, but City have better offensive weapons. But the reality is, Pep is the GOAT tactically and Rodri can dominate a match and lock up midfield. So if Arsenal and City went head to head and were both at 100%, City would most likely win because of those factors. But because the PL isn't based on 1 match, Arteta is trying to boost the average level of performance in each game so we eke out an extra point here and there. If that's not enough to win the league, so be it, we're up against a fantastic team. In fact, ok, I'm a Gooner and think the invincibles were an unbelievable team with some all time great players but they never won the CL or a treble. So on that basis this City side is the greatest PL side there's ever been. The facts back that up.
So in that context it's like being the guy who finishes 2nd to Usain Bolt - sometimes if you've done your best and maximised your performance and you still can't beat the opponent then you have to respect that.