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 I think that the very idea of personal guardianship of individual players has long been outdated and no one uses it - everyone plays zone control.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.