Now the important question: has any band ever released a single album that was every bit as good as the albums during their "golden age" after a string of mediocre albums?
Yes.
1 -- Elvis Presley: He was a genius of singles in 1954 to 1961 or so, then completely sucked from 1962 to 1967/68. After "Guitar Man", "If I Can Dream", and the '68 Comeback Special, he returned from the dregs to make
From Elvis in Memphis (1969), arguably the greatest and most critically-acclaimed album of his career. The ones that followed until about 1972 were also great.
2 -- Roy Orbison: Lost the plot amidst changing musical tastes and the unbelievable bad luck of his personal life somewhere around the late 60s. Had an incredible comeback in the final two years of his life with the platinum-selling Traveling Wilburys album and his last record,
Mystery Girl, which was his biggest seller ever (and had a great song by Bono and Edge).
3 -- Bob Dylan:
Time Out of Mind (1997) and
Love and Theft (2001)
4 -- Paul Simon:
Graceland (1986)
5 -- Steely Dan:
Two Against Nature (2000)
6 -- Bonnie Raitt:
Nick Of Time (1989)
Does anyone who became a U2 fan before year 2000 honestly think any of their post-2000 albums every bit as good as any of their golden age albums?
I became a U2 fan in about 1987, and I think ATYCLB is an excellent record, and that HTDAAB is their 4th or 5th best album. So the answer to your question, again, is "yes".
Every rock/pop/R&B-based recording artist will inevitably have a peak period of inspiration and creativity, and then a decline or a dry-period, or at least a lessening of inspiration. For some people, the inspiration and drive comes young and goes quickly (The Sex Pistols, Sinead O'Connor); for others, it lasts a long time (like Prince, from 1978 to 1987 or so).
U2 have somewhat managed to buck the inevitable trend of decline by a few factors:
1) They're very un-prolific: none of their releases are ignored or pass the public by (this is unlike artists who were aging in the 70s and 80s but still put out an album every year of increasingly diminishing returns). But even U2's younger peers are way more prolific than they are. They design every move they make into a big EVENT, guaranteed to get everyone's attention.
2) The 4 guys have all been 100% committed to the band's cause from the get-go until today. They're all extremely passionate about what they do, and they're highly self-critical, never falling into the trap of being satisfied or arrogant about themselves. I honestly cannot say that about any other artist I can think of who was commercially successful for 20 years or more (or even 10)...
3) The older they get, the more aware they are of the need to keep up with the changing times. U2 have always been the first to embrace new technology, new young bands, and new ideas. Again, this is the polar opposite of the earlier generation of 60s bands who were still pretending it was the 60s in the 80s and 90s (see: Crosby, Stills, and Nash).
I don't know if it's even possible for a rock band in its 40s (soon to be 50s!) to be as "good" as vital and essential as they were 20 years earlier, any more than it's likely I'll be as good a sexual performer at 50 as I was at 25. But, you know, I think U2 are as close as any band has ever been at their age to being as good as they were in their 20s. Which is pretty amazing.
Sometimes, we can get hung up too much on subjective judgments like "better" and "worse", which really are meaningless. I think what does have meaning is things like "level of commitment" and "different". That is, as long as U2 are 100% committed to what they're doing, and as long as the music they make is not a tired re-tread of past glories, I will always enjoy what they do and I won't feel the need to compare them to their past. Yes, maybe it'll never excite me as much or feel as culturally relevant as it did in 1988 or 1993, but that's okay with me. As Oscar Wilde said, "Youth is wasted on the young."