Just a correction on the very first post in this thread... "Pop" did NOT go 3x Platinum in the U.S. If it had, we might not have seen AYTCLB (interesting thought...). Rather, "Pop" went 1x Platinum in the U.S., which is U2's lowest total in ages. Actual consumer sales are around 1.5 million (not enough to get it to 2x Platinum).
While there are many artists - even former big name artists - who'd love a Platinum album, by U2's standards, this was very low. Additionally, after "Pop" U2 had pretty much done all they could on the "experimental" level. They had been doing this from 1984 to 1997 - pushing the boundaries again and again. It's little wonder they decided to experiment not with new sounds so much as with their own sound. That said, given how much U2 has experimented, does U2 really have their own sound? If so, which is it? "War"? JT? AB? Hmmm...
I also take issue with the last two albums being "safe". Even Edge's own daughter questioned the songs on HTDAAB as she asked her father if he had listened to the radio lately. This is a key point - the songs on ATYCLB and HTDAAB are quite unique. JT is a powerful album - not that risky at all, compared to UF. It has more straight-forward songs. But it stood out at the time because there was NOTHING on the radio like it. Even Rolling Stone magazine and U2 didn't think it would do *that* well (estimating that, at best, it might reach 3x Platinum). The same is now true for HTDAAB. The album simply doesn't blend in at all with current hit music. It's more of a surprise than an expectation that Green Day and U2 had hits in this environment! Rock music simply isn't represented.
So while these last two albums are admittedly not as "out there" as past works, they still are experimental ("Love & Peace", "Fast Cars") and they still are quite unique compared to popular music. If anything, U2 made the songs a bit easier to accept. But then, that's something they've done forever. What makes U2 so great is that they have the ability to experiment, be unique (even if they are influenced by others), yet have their work be something the mainstream would enjoy. This is their main reason for their longevity.