Gaga didn't lose much of her fanbase
BTW didn't sell 800k+ because of Gaga's huge fanbase. It sold that much, because it was a cultural event.
On the other hand ARTPOP sold almost as much as PRISM which had smashes and was Katy's biggest opening week (because GP bought it). However, GP wasn't that interested in ARTPOP, but Gaga's big fanbase compensated. That's why PRISM - 280k AP - 260k
So basically more of the GP bought PRISM first week than they did ARTPOP, but Gaga's fanbase is bigger.
Declining sales doesn't really count when we're talking about first week sales of huge artists.
Gaga's first week is going to exceed 200k with ease.
Imagine if the lead single really clicks with the GP. GP + fanbase, Gaga will easily top ARTPOP's opening.
Y'all are still forgetting how much was against Gaga during ARTPOP. Also the promo schedule was ****. She had only 1 appearance on SNL during her first week, and did that artRAVE thing

And from September to November Gaga didn't do anything in the USA promo wise. And fired Troy 2 weeks before the album release.
Good music + Good promo (not saying more necessarily, but better promo) + GP + big fanbase >>>>> Big fanbase, music that didn't click with GP, bad promo schedule, messy Gaga