The main reason is that after first few weeks, like maximum of 10 but probably less, teams will realize that Haaland is not their only scorer, and City is using him as bait, you fear him scoring, so others score easier. Then teams will end up defending normally, and that will make Haaland score a lot easier as well. That's how I think the season will go, first 2 months not that great, after that it is all goals galore for him until the end.
I see and understand your logic there. Haaland might just continue to score more goals for Manchester City. Defenders would be automatically drawn to seriously mark him as he’s an obvious threat and capable of scoring goals. Well, so are almost if not all the players in the first squad of Manchester City.
Everyone is a threat and capable of scoring goals but perhaps some could be considered more of a threat than others. Manchester City is that good. They don’t actually need to rely on Haaland to score goals.
A coach of any club playing a match with Manchester City should know that while Haaland is the striker, center back players also scores goals as well.