I think the biggest problem is that rock music hasn't changed at all. Almost every new band I hear on the radio is straight-up ripoff of an early 90s band with improved production value. I think it's less that it didn't age well and more that it's across the board fatigue with the style of music.
Isn't it all, though?
I remember flipping through my dad's numerous The Ventures LPs and finding a compilation of surf rock anthems. Holy shit were there a bunch of surf bands back in the 1960s. The ones I remembered from the compilation were Sandy Nelson, The Bel-Airs, and The Rivieras, with other British-invasion bands such as The Zombies. I started noticing a lot of similarities between the groups: the twangy reverb on guitar, the start-and-stop drumming, and the Beach Boy harmonizing. There was little original space between the songs of the bands.
My own theory to this "fatigue," as Emp explains it, can be summed up with the analogy "beating a dead horse." Well, bands were literally beating a dead music form after Elvis, the Beatles, the Rolling Stones, the Beach Boys, Hendrix, Zepplin, Deep Purple, et al. They were simply recycling every musical quality like a player piano performs the spool of sheet music it is given. The instrumental formula stays the same with many bands: lead guitar, bass, rhythm guitar, drums, and vocals, hardly breaking the mold or introducing new instruments into the fold. Until bands start to understand that playing rock 'n' roll, metal, alternative, punk, or jazz is incredibly repetitious, music will never break new creative ground.
As for the thousands of spin-off bands, I assume that these are bar bands/hometown hero types that put their faith in the local fan base for support. I've been in numerous bands and nearly all of them stayed within the confines of the region. Most of us were dirt poor and couldn't tour, but a few of the bands did small southern US tours and had a bit of success with different venues. One of the bands I played with did nothing creatively new. The guitarist was fixated on the e-bow (an electronic contraption that when put on a particular string(s) will make it reverberate as if you were using a violin bow) which almost every band was incorporating during that time. We were taking our cues from bands like Malady, City of Caterpillar, Pg. 99, Mogwai, and Death from Above 1979.