Well first of all he has to do all that, which is far from guaranteed.
Secondly, it is amazing how short people's memories are. The only reason people are claiming the Ballon D'Or for Haaland (he is still behind Messi in the odds BTW) is because he is what is freshest in people's minds (and also because he plays in England and English football is uppermost in the mind for the vast majority of people in this forum).
Let's say that City won the treble and Messi had the season in France he had, potential French player of the season honours, whatever, but failure in the CL. But instead of being in December, the World Cup was in the summer. Argentina win, Messi wins the golden ball, silver boot etc. Messi would then win the Ballon D'Or pretty easily because it would be freshest in people's minds. Instead of people complaining about it just being 7 games. It was one of the best World Cups ever with probably the best final ever, which was watched by 1 5 billion people. In that scenario, it would be City's success that would largely be forgotten about.
Also, whilst I think Haaland would be a worthy winner if City finish the deal, I don't think his all round performances actually merit the kind of 'closed case' noises that people are making. As I and many others have said, for all his goals, a lot of the time he's not even the best player in City's team when you actually watch them play.