Haaland is far more than an additional striking strength. When Haaland plays the other team has to dedicate defensive capabilities to stop him, which impacts the team as a whole. Having a really strong striker within your line-up is more impactful than just having this striker. An opponent is forced to think about it and come up with a match plan that includes this striker of course.
But the discussion whether De Bruyne is more important or Haaland, I think that also depends. There will be days when Haaland scores that one goal needed for the win and there have been days when De Bruyne scored it, like against Real Madrid in the Champions League. But one thing is for sure and that is De Bruyne's injury is among the worst issues that Manchester City could now have.
I'm not sure if this works with City this way, although for many other teams it would be 100% true. It seems to me that City are more dangerous with ball control + pressing than those who play in their attack. I think that the very idea of personal guardianship of individual players has long been outdated and no one uses it - everyone plays zone control. It doesn't matter if Haaland is in the line-up in separate City games or not, the opponent will play his own game formation.
I agree with most of this, but when you take the game one in the Champions League between Manchester City and Real Madrid as an example, it was obvious that Real Madrid paid specific attention to taking Haaland out of the game and they were highly efficiently doing that. Haaland was dead in the water that game. Though you are right that zone control is the way to go, but there is also some dual effect when it comes to taking out Haaland. You never want him to have too much space to enfold, so that is one way to take care of him. Keep the space small for him. The other way is to take out those players who assist him mostly and that is De Bruyne. By not letting De Bruyne get too much space to shoot precise cross balls, you kill two birds with one stone. I am looking forward to all the Champions League games as I think that this year Manchester City won't be as superior as they were last season. There is no hard evidence yet for that thesis to be backed up, but I feel like Real Madrid will play a lot stronger than last season.