Thing about going to Bayern is that, yes, it pretty much guarantees him a trophy; but is it really a trophy that means much of anything these days? What is it? 11 in a row? The procession the BL has become has made titles for Bayern almost meaningless at this point. If they don’t win the title it’s a massive failure, and if they do it’s just the expected and most predictable outcome. So he’s essentially playing for them for the chance of European glory. Kind of like what signing for PSG means.
In Europe, you think, with Kane they would stand a much better chance. But watching them last season under JN and then TT, you got the impression they are miles off winning the CL. Even with Kane. Now, coming to United isn’t likely to put him any closer to the CL trophy - I fancy United’s chances with Kane to be reasonably comparable to Bayerns’. And he wouldn’t be guaranteed a league title. In fact the best he could hope for would be a title challenge as an underdog. But he would have a shot at picking up a trophy or two in the hotly contested domestic cups, and he could keep up his quest to become the all time leading PL scorer.
Personally I think going to United keeps him much more relevant at the top level of the game. The BL is currently lacking in any credibility as a top league. Bayern, and their lopsided financial dominance, has killed that competition.