This thread is funny
But seriously, aside from specialising in a completely different genre of music, I think it's pretty weird to compare an act who has the sum total of two singles and one album under her belt with a band with a vast discography and one of the best selling albums of all time, who have been around for more than 30 years.
Ten years ago you could have written the same post and replaced Lady GaGa with, say, Alanis Morissette. Alanis was very "relevant" back then, now, to put it mildly, not so much.
Anyway, I'm with Gerald on this one, I don't think it's got that much to do with productivity. I'd say Johnny Cash is still relevant, and he's not even with us any more. But he's so iconic, his music is still played and his influence is so huge it would be silly to say he's irrelevant.
As it is more and more new bands are coming out and claiming the Mac as an influence from The Decemberists to Bat For Lashes and Ladyhawke (all very "hip" and relevant artists) and the band is still out there and getting plenty of press and attention, maybe not the kind that Lady GaGa is getting but I'm pretty sure the next big thing will come along and steal some of her light too.