This happens I believe with every band that has reached massive stardom and they have lived enough to enjoy the super rock star status for a while. The Stones, U2, etc, they had all made huge contribution to rock music, and the truth, they became huge rock stars DECADES ago and people expect to continue delivering ground breaking record. But for these bands, their own past if their greatest enemy. If U2 pulled a second Joshua Tree or Achtung Baby, fans will say they are rehashing the same old stuff. If the pull an experimental, ground breaking record, people will say they have lost it, without even resembling what they once were. This happens across all genres.
For example, Metallica, a band whose career is in my opnion very similar to U2's, despite being different genres. They became the greatest metal band in the 80's, in the early 90's they became more radio friendly and became one of the world's greatest stars, late 90's they became more experimental and the "hardcore fans" became disappointed. In the 2000's they tried to return to their roots, and Death Magnetic had some really great metal songs, but fans thought they were trying too hard to emulate their earlier success.
When a band becomes so big as those example, they will NEVER be able to reach the expectations set upon them, no matter whey they do, their fans will never be completely satisfied.
I doubt U2 lose much sleep over satisfying their core fanbase. They know we've stuck by them for long enough that we'll buy whatever they put out, and we'll stump up our cash to see them whenever they tour. The loyalty of U2 fans is really beyond question at this point. We're not all going to suddenly disappear if they decided to push the envelope and fell on their ass (in fact, a lot of us would laud them for making the attempt). What U2 probably do worry about is: whether they have the radio hits, whether they can ensnare young fans, whether they can be critical darlings with the Grammys shining on their mantlepiece, and whether their album can shift X number of units. The existing fans aren't a problem. U2's gargantuan expectations for themselves at a period in their career when their creative prowness is failing, is the root of their consternation IMHO.