I feel, to a certain extent, every U2 record is a stab at relevance. They want to be relevant, they want to be commercially successful. They always have. They were disappointed that Pride didn't chart higher on the top 40 in 1984. This isn't a band that would be content releasing records that they were proud of, that got great critical reception, and sold 5,000 copies each. Pop and ATYCLB sound very different from each other, but they both want to be relevant. It's just that they felt that Pop ended up not being relevant(as has been said, it was relevant outside the U.S., but relevance in the U.S. market is very important to them), so they tried something else. The reason ATYCLB is perceived a certain way is that U2 always tried to see how far they could go, how deep their exploration could go, how far they could push their own boundaries, while still remaining commercially successful, but ATYCLB was seen as an attempt to be commercially successful by going back to their roots(I personally feel HTDAAB fits that perception better). As far as a regression...the only record that I think you could use that label on is HTDAAB. I don't the label fits ATYCLB all that well. It's not like Stuck In A Moment or In A Little While are the most U2-sounding things ever.
My own personal feelings are that I love Pop, it's in the top 5 for me, if I'm listening live. I also love a lot of the tracks on ATYCLB - Stuck In A Moment, Kite, In A Little While, Wild Honey, When I Look At The World, New York(the live coda really gets me), The Ground Beneath Her Feet(it was the 12th track in some places, the music was written in the same general time period as the others, and it was played on Elevation, so I consider it a part of the record). I don't dislike Beautiful Day, but it's just played out at this point, and I'm a little bored of it. Still, as much as I love those tracks, the record as whole doesn't create a universe like Pop(and so many other U2 records) does, and it doesn't pack the punch, so it's still probably in the lower half of my rankings.
As an aside, I often wonder what might have happened if U2 had held on to HMTMKMKM, put it on Pop instead of giving it to Batman Forever, and released it as the first single. It was a successful single when it was released in 1995, it probably still would've been successful in 1997 and, maybe, just maybe, would've been the initial punch the record needed to sell in the U.S. Also if Gone and DYFL had been released as singles instead of IGWSHA and Mofo(love them both, but they weren't the right choices for singles). Imagine if the singles had been:
HMTMKMKM
Gone
DYFL
LNOE
Discotheque(without the pressure of being a lead-off, maybe it would've fared better later on)
SATS
Maybe things would've been different...