I can guarantee you there hasn't been a single player in the history of Manchester United, but possibly in the history of the Premier League as well, that thought, upon signing for his new English club, that "if I do well here, I can get my move to Bayern and go on to do bigger things there". None of the top ~6 PL clubs are stepping stones to Bayern Munich. You go to these teams for the long-term, especially to the likes of United, Liverpool, or Arsenal.
With Madrid and Barcelona, it's obviously different, but Nordic players tend to like England and the Premier League way more than players from Southern Europe. And it's not like the likes of Christensen, Rüdiger, Alaba, Lewandowski, etc. kicked up a fuss at their clubs in order to go to Spain. They remained patient and respectful with their clubs, if I'm not mistaken. And only left when there was nothing more to achieve at their clubs either, as they had won everything there was to win and they ran their contracts down.
The reality is that if United were a top 3-4 team in Europe that was challenging for the league and CL every year, Haaland could've been attained and retained for a long time. That's why he went to City, for sporting reasons. So what United need to do is become a serious club again sporting-wise, and we won't be a stepping stone to any club.