This is always going to be the default pattern, songwriters in bands usually seem to have one or two albums worth of great stuff with a maybe a smattering left for the third album. Successful bands have long moved on from the all-living-together-in-a-rehearsal-room camaraderie and are brought to the studio in their separate limos, the coke that made the early tours such fun and fuelled those legendary all-night sessions on the second album is now an obstacle that hampers creativity etc…
Be easier to think of bands who either managed to keep the quality high for a longer period (Beatles, Stones, The Who, Pink Floyd, Kinks, Zepp, arguably New Order) or either started off a bit rubbish and did their best work a few albums in (U2 being the obvious example, maybe Floyd in this category).
That said, my list of the most heinous offenders of the OP’s premise would be:
Red Hot Chili Peppers
Jane’s Addiction
QOTSA
Metallica
on the basis that it is, in all the above cases, quite hard to listen to their latest albums and hear how much the vibe of their early work, that made them great, has entirely gone.
I wonder what the thoughts are about bands who may not have hit the big time initially, but then did some way down the line, usually with diehard fans complaining that they’ve “sold out” and that their new stuff isn’t a patch on the old stuff. Bands I’m thinking of here include:
Simple Minds – Honestly, their early stuff is, IMHO, a million times better than the “Don’t You Forget About Me” era stadium landfill.
Black Keys – Hugely massiver then they have ever been, but I’m not alone in preferring them when they were like the White Stripes with a competent drummer.
Orchestral Manoeuvres in the Dark – representing the well worn path from artistic, creative iconoclasm on early records to “you know what, some massive hit singles might be nice” pop success.
However, all in all the award for “band formerly most brilliant and now most rubbish having jettisoned everything that made them great” surely can only go to the Chili Peppers?
