It's worth considering but also United's team significantly dropped off after that 2003 season, Scholes spent a lot of time out with an eye injury, Keane had the hip problems and then eventually got sacked. A lot of the time United were playing Phil Neville, O'Shea in midfield, a young Fletcher. Kleberson and Djemba Djemba failed miserably. There was also the keeper issue between Barthez and van der Sar, and Rio Ferdinand got an 8-month ban for missing a drugs test. It feels harsh to blame Van Nistelrooy for those problems for the team not being as successful.
As for Haaland, City are very successful with him, they won the CL without him and a treble, won the league this year so you could hardly say they're getting worse. Better with or without him? Probably not all that different, gain in some ways, lose in others without him. Do they win the game yesterday with Alvarez up front on his own instead of Haaland? I don't think it makes a difference.