This isn't true.
2018-19, KDB was missing for almost the entire season with injury yet City went on a crazy win streak to reel in Liverpool and win the title.
2019-20, KDB had an outstanding season for City, who finished 18 points behind Liverpool, due to Rodri getting used to the pace of the PL and City being unusually porous to counters.
KDB offers City an extra lethal edge, but their DMs (Fernandinho, Rodri) and 8s (Gundogan, B Silva) are way more important to City's dominance, both in solidity and progression of the ball to the final third.
I'd give you that Rodri is as if not more important, but an element of that is because they have no one who is nearly adequate as a back up. Where as they have plenty of creative players.
What you see when De Bruyne is missing though and now also Haaland, is teams are a bit less scared of them and start to realise they can counter and hurt City if they break through the initial press, because Rodri can't be in 2-3 places at once, no matter how good he is.
Silva, Gundogan etc. Are/were important because they will actually get in and about Rodri so he isn't left on his own to cover stupidly large areas, and will drop back in possession so Rodri isn't the only thing linking the defence to the attack. This is something Ten Hag and United fail to even really attempt to do, and why whoever is in Casemiro's role this season looks like they are drowning. The idea Casemiro in the space of a few months has gone from one of the best players of his era in this role, to being completely crap at it, is silly. Last season he had Eriksen or Fred covering half the width of the pitch for him. This season he's had no one. I can't think of any single player whk you could put in that position in the current United team, with how ETH sets them up, who wouldn’t struggle badly.
Which is the point I'm making with Rice. I think he's done fine but if Arsenal are serious about winning the league, playing Havertz next to him is going to be a problem. City barely get away with it with Rodri and not only is he better than Rice but City also have better players and are more disciplined into playing that kind of set up than Arsenal are. I think it's been evident in most of Arsenal's games and now it is starting to affect the results. They've had others like the United game where only marginal moments of luck have saved them from giving away cheap points due to their midfield being too open.
I only brought Casemiro into the equation because i think he's got a lot more to his game than Rice, and yet struggles when put in a similar situation (albeit not helped by the whole team and tactics at United being a garbage dump).
Some of the comments in this thread make me think Arsenal fans see Rice as this magical younger super Casemiro/Rodri type midfielder, because that's what their current set up would need him to be....but not only is he not this, I'm not really sure who is. Ultimately I think Arteta needs to change his approach and Havertz needs to be repla ed by someone who'll support Rice more....because from what I've seen the rest of them are hard working and disciplined enough to make it work.
Again I'm not comparing in terms of who is or isn't better here, but Carrick is the best United player I can remember in the Rice type role, and he would always have a Scholes/Fletcher/Giggs/Anderson/Cleverley in the team with him. Someone who would drop back into the hole rather than spend the game acting like a second no10, and who wouldn't randomly disappear for 10+ minutes every half.