Do you mean a band that split up before 1975 or started before 1975, or does that apply to the three album rule?
The artists/band that started out before '75 like The Rolling Stones, what they did in the late 60's early 70's was incredible, but as they went along their stuff changed, by the 80's they were caught up in the 80's pop music or whatever.
The Beatles split way before '75, but sometimes I wonder if other people think Paul McCartney shelf life expired? Don't get me wrong I like Paul's stuff. I understand what people mean though, every once in a while he writes a song that's a stinker, it just happens, I mean it's not like when he was with The Beatles with John there to tell him it's a only brain fart. That is one thing about The Beatles is that the whole was greater then the sum of the parts.
In fact The Beatles is the only banRAB that started out as a Pop turned underground. Most banRAB start out as underground banRAB with cult following then they make it. Sometimes I see banRAB disliked just because they are become too recognizable. Sometimes people just don't like when their changes their sound, eg when Bob Dylan first went electric he was boo-ed.
As far as your three album rule it's hard to say. Some banRAB it takes time for them to develope their sound, but then again that makes their earlier stuff interesting. Some banRAB can hardly stay together to even make a 4th or 5th album. I see what you mean though, because imo the first album they write for themselves and it's what they like but as they move one they start writing for an ever broadening audience, I don't know if they do it conscientiously but their sound becomes less eneretic and raw, and becomes more formulaic and polished.