Also it's a bit premature to be talking about Gaga's longevity in 30 years time when she had a peak of 3 years and a decline of 3 years. As Cain pointed out, her HUGE debut, peak and overexposure so early in her career was both her biggest achievement and her biggest curse.
You can't talk about her longevity like it's set in stone just because she's your favourite. That's not realistic. Especially when already-solidified legends like Mariah Carey and J.Lo are struggling to compete with today's girls. In 15 years time in the year 2029, all the little gay boys who are only just being born now won't be stanning for Rihanna or Katy Perry or Lady Gaga. They'll be stanning for some future 18 year old pop star who'll be making all our faves look really old
I think everyone underestimates what an achievement it is that Madonna's still going so strong in the game, but that's because she was the first and the original. We all expect that just because Madonna's done it, our faves will too. Which in some situations is weird, because a lot of people make ageist jokes about Madonna but then expect their fave to be doing the same thing at her age. But it's not guaranteed. I think all of our faves will come to have an expiration date. That won't stop Lady Gaga from always performing. She's a born performer. But I doubt in 30 years time when she's 58, her legacy will be so huge that she'll be continuing to sell out arenas and stadiums. She'll probably do a Vegas residency and start touring smaller, more intimate venues as she gets older.