Wow. This is a GREAT discussion. Lots of good insight.
I knew that U2 were technically challenged to some degree, but I guess I never gave that much thought to their limitations. I always assumed that, given how much $ they make and how much adoration they receive, they (as professionals) could reasonably put the requisite time into their craft to be able to authoritatively deliver a repertoire of 50 or more songs.
After all, most of them are not that complicated, musically. And the ones that are, are either supported by recorded tracks, or augmented by "invisible" musicians.
My point is that I think if the band really wanted to do varying sets, they could.
Also, there is no reason, given the thematic similarities of many of their songs, that they couldn't craft totally different sets that "say" the same thing. And I feel such collages would be stronger because of their individuality.
There's something to be said for art that is "just exactly so," with every piece perfectly in place. But U2 has never been about perfection... it's not their forte, and they've always been at their best when willing to make mistakes and reach for something extra. And the current set is by no means "perfect." So my preference would be for them to reject the studied, ostensibly "just so" setlist, and opt for different lists that challenge them and their audience
Part of what will establish U2's true artistic legacy is the risks they take. They would not be the immortals they are today without the risks (and subsequent rewards) of Achtung Baby and ZooTV. They'd be a nice relic from the '80s, a cut above the Alarm, Big Country, or other contemporaries.
There is already no band without a 20-year headstart on them to rival them commercially. Where U2 should be focusing all its energies is on creating art that will last. And lasting art is about challenging perceptions, not about regurgitating past triumphs.
Frankly, U2 is probably already "better" than the Stones, but they may never be "bigger" than the Stones, as they'll never be bigger than Elvis or bigger than the Beatles.
The Rolling Stones played a key role in the development of rock and roll, during the artform's first 10-15 years. U2, like all post-Buddy Holly, post-Elvis, post-Little Richard, post-Beatles, post-Stones, post-Who bands, is coloring with someone else's crayons. They can't go back in time and have that seminal impact. Their charge is to push the artform.