Ok first of all do not compare Gaga with Britney
Britney has had constant success throughout her already 15 year old carreer. And this is her 8th album, and she doesn't have ANYthing to prove to anyone.
She can flop all she wants - her longevity is more than most girls can dream about...
The reaction atrl is having towards this is because before, some lil monsters were always laughing at girls with smaller sales, and some of them were very cocky and were constantly "warning" everyone about the slayage that Artpop will do etc etc
And of course also Gaga overhyped her album and now it's underperforming (severely) especially since it was jokingly "the album of the millenium"
