Sword/Shield are going to sell 15 million+. People freaked out about Let's Go last year and it will end up selling ~12 million when all is said and done, which will put it at the same sales as FireRed/LeafGreen and HeartGold/SoulSilver.
Yeah, people did freak out about Let's Go last year. Like, 13 people.
This isn't the same situation at all, this is a big deal and will lead to at least 1 or 2 million potential sales lost... but, then again, I'm confident that there will be a lot of sales gained from kids and casual players hopping into the series and just generally people who don't care. So, yeah, it'll do 15m+, maybe even 20m+, and it's a shame - it's the reason this franchise is doomed in the first place.