but were not one of the 15 people who were in the band the longest.

I’ll go first: Duane Allman

Are there any others? If not, what number of years do we have to drop it to to find someone else you can think of?

After typing that, I thought of Janis Joplin in Big Brother and the Holding Company (which I had to look up to confirm). But they don’t really work, since lots of people only know their songs as Janis Joplin songs, and not all her hit were with them. And they weren’t legitimately famous without her, although that wasn’t in the original criteria.