Just trying to think of musical artists who have not only been around a long time but have participated in other successful bands/ acts or been aligned with other musical talent.
Eric Clapton and Kris Krisstoferson come immediately to mind. Probably also Paul McCartney.
I know the question is wide open but who else?
Kris Kristofferson? Other than writing “Me and Bobby McGee” and being in A Star Is Born with Streisand, what’s he done?
I’d vote for Nick Lowe and Brian Eno.
Bruce Springsteen
Prince can still rock the house (I saw him at the O2 arena last year) and has been around for a good while. His songs have been performed by various others over the years. He hasn’t done a really great album for some time though, I suppose.
Rita Coolidge may be a starting point. He has been with a lot of musicians over many years and is still kicking.
U2.
Classical artists as well: Bach wrote great music for well over forty years, Mozart would’ve were it not for his early death at 36 (even then his career of writing music that is still played today was almost 20 years (dating from his 25th symphony (the opening work in the movie Amadeus), which he wrote when he was 17)). Beethoven, Brahms, etc, all had long, productive careers.
Bob Dylan (primarily a solo, but he was with the Traveling Wilburys).
Steve Winwood, successful with The Spencer Davis Group, Traffic, Blind Faith, and as solo. He just released a new album.
Brian Wilson and Ray Davies are both still active as solo artists, and I think each was in some obscure group or other back in the sixties…
Tony Iommi, the only constant member of Black Sabbath. There’s a whole scene dedicated to aping his sound.
Ian Anderson
And the Band. And collaborated on an album with the Grateful Dead, as well as songs with Joan Baez and Emmylou Harris, among others.
And Verdi.
Another one: Django Reinhardt, the first lead guitarist. Before he came along, the guitar was purely a rhythm instrument.
Neil Young; especially with Buffalo Springfield & CSNY
Didn’t Musicology win a bunch of awards?
BTW, I agree. Prince has been enormously influential in ways that go well beyond the covers people have done of his songs. I won’t go so far as to say he singlehandedly created the Minneapolis sound, but it definitely wouldn’t have gotten as widespread as it did without his talent as a composer, arranger, and producer.
David Bowie.
Prince is the first person I thought of, too. Glad to see others have noted his influence.
I’d nominate Sonic Youth for alt/college/whatever rock. They fit the first criteria of the OP, longevity-- they’ve been around for 27 years-- and are quite known for the second criteria of the OP, influence. They “broke” Nirvana into the mainstream (encouraging DGC to sign them, touring with them, etc.), which completely shifted the direction of popular rock music. (I’ve heard more than once that they remain on DGC because of this, not because of their own success). A few of their albums continue to be considered classic, and the decade+ old Daydream Nation managed to pick up plaudits on its deluxe reissue recently. In the 90s, they tended to be called names like “godfathers of alternative.” They prodded Spike Jonze into directing his first music video for them, arguably bringing the former skate videographer into his current career of movie/music-video auteur. Their promotion caused bands like Shonen Knife to become semi-popular acts, rather than novelty import acts. IMO, they’re the influence for bands like Pavement.
ABBA and their success with Mamma Mia.
Elton John helped the “Disnification” of Broadway. If the predicted success of next season’s “Billy Elliot” comes to pass, he will have become a major player in musicals.
Kate Bush has only been relasing music since 1978, but her influence is vast, deep and diverse. She has to be one of the top 5 influential female artists ever (that figure was plucked from the air, but I’m certain it’s close). She hasn’t only influenced other female artists, but male musicians ranging from Jeff Buckley to Big Boi (Outkast) to Tricky to Tupac to Rufus Wainwright have cited her as an influence too. She’s also influenced a generation of writers, painters and poets.
I just picked up The Fall’s latest - ‘imperial wax solvent’ - very good. Always different, always the same 
Huge longevity, being going since 1976. Mark E. Smith is a unique British music institution like no other, really. There is literally no one you could compare him to. They’re fairly influential I suppose, the aforementioned Pavement have a big Fall influence. They’re one of these groups that are seen as quite avant-garde but actually have a mainstream pop orientation as far as their sound goes. THat being said, I have never, ever heard anyone attempt to sing like MES.
Where is your mum-uh? Your power is gone-uh!