Haaland won't join Manchester United because he will have to share his striker position with Ronaldo. Even Ronaldo won't have too many seasons left but 1 to 3 seasons are too long for waiting with Haaland. He and Mbappe know that it is their time because Messi and Ronaldo gradually step downwards. They are hurry and don't have patience to wait just a couple of seasons.
Yeah, Manchester United have Cristiano Ronaldo as their main striker now. But I don't expect him to be very beneficial for the team every season even if he plays for a few more years. Manchester United will need a young and talented striker for sure. And Erling Haaland is the best candidate for this. He is only 21 years old now and his performance is way ahead of his age I think. Normally, a striker starts having a very productive period between the ages of 25 and 29. But I think that Manchester United won't be able to sign Haaland while there are teams in the race like Real Madrid and Manchester City.