I could see it hitting 20 million. If BD/SP sold like 13+ million just over the holidays, and I would certainly expect a brand new fresh take on Pokemon gameplay to outsell a remake, then PLA can definitely hit 20 million lifetime.
I've only ever played Pokemon Red and Sword. Red was awesome back in the day. Sword was alright but felt like the series had barely evolved since Red, and it also wasn't as difficult and of course the novelty of Pokemon was gone after two decades. Sword was decent but felt like the series was just going through the 20+ year old motions. PLA ain't perfect but it feels fresh compared to Shield and it's what I would have assumed Pokemon would have been like when the series finally hit a console, rather than what Sw/Sh was. Now I just want to see them keep building on the PLA gameplay, fleshing out the experience beyond this good first step.