Ironically, SAF wanted Lewandowski back in his Dortmund days but they ended up getting RVP and Lewandowski to Madrid is one of the longest running transfer rumours ever.
Coming to Lewandowski's future, if he were to leave Bayern:
PSG: Doubt he'll go to the French league.
Chelsea: Just splashed 100 mil on Lukaku.
Juventus: Just got Vlahovic.
Liverpool: Against club policy plus won't wanna afford his big wages.
That realistically leaves Barca, City, United and maybe a surprise club.
I think he'll be a great signing for United. People have been predicting his decline for years now(look at the 2017/2018 CAF verdict on him) but the man only gets better with age like fine wine.