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.
Gamers on forums/twitter don't realize that franchises like Pokemon, Call of Duty, and FIFA are driven by casuals, and casuals like more of the same. Most COD players only care about playing online with their friends, so the announcement of no single player campaign last year didn't bother the majority of players. COD still ended up being the second best-selling game in the USA, only behind Read Dead.
The majority of Pokemon players care about the single player experience, catching/raising Pokemon, battle trainers, progress through the story, and defeat the elite four and become the champion and hero of the region. All of that will be Sword/Shield.