There is certainly a mainstream and U2 is bedded in it. It's just that a lot of modern rock, or at least the more credible (artistically) is absolutely niche.
And these opinions, at this point, are like fans wondering, wishing and hoping when Michael Bay is going to stop being Michael Bay. U2 are who they are. I've echoed the above sentiments for years and years. At some point you just have to accept who U2 have become...and currently are.
At least attempting to fuck up the mainstream is a path to a more favorable outcome given their stated goals concerning relevance. If you're going to try to have "hits" (which I certainly don't care about), if you're U2, you might as well try and do something unique or interesting with it. I'm not talking about any of our fan idealism. So while I agree in principle with all of the above about what U2 could do, let's be real here. This is something U2 covets and these kinds of opinions won't change their minds.
U2 are the biggest rock band in the world. Maybe, by far.
Everything they do is mainstream. They just had the biggest concert tour in history...with an album that had zero hits and one that didn't sell particularly well. You can only do that being a mainstream staple.
They do a Broadway musical (and who gives a shit about those outside of a niche community?) and all of a sudden it is punchline fodder everywhere.
So yeah, while I agree with the fan idealism in spirit, I'm not deluded enough to think U2 is going to make their version of 'King of Limbs' or whatever. Hopefully they will...but I severely doubt it. It's going to be more of the same in a lot of respects, and if so, it might as well try to echo Zooropa, rather than ATYCLB.
Bono's vision is preferable to the rest, who seem content to play it more safe ("brick and mortar" is an apt way of putting it, Andrew, I agree. They are old fashioned). But either way, U2 will remain 'mainstream' no matter what they do. They have passed beyond the point of being irrelevant, which is why their goals about relevance are so idiotic...but we can't do much about that at this point. All I hope for is some great music. I think the most plausible path to that, is if they listen to Bono. That's all I'm trying to say.