It will become the best selling Poke'mon game period. Switch should have a lifetime userbase that is about 30 million more than 3DS. Because all of Nintendo's games are on a single system now, those Switch users will stick with Switch far longer than they stuck with 3DS. A lot of people really started to shelve their 3DS by 2015. That's not going to happen with Switch, because we are getting both handheld franchises, and console franchises on the same system.

Sun/Moon sold 23.55 million units on 3DS. Ultra Sun/Moon is just a GotY version of Sun/Moon. Refusing to count the sales of Ultra Sun/Moon is like refusing to count the GoTY edition sales of Fallout 3, or Witcher 3.

So Sun/Moon did such amazing numbers on the 3DS, when the 3DS probably only had 50 to 60 million active users out of its 74 million lifetime sales. Meanwhile Switch will have 100 million or more lifetime sales, and 90 million of them will stay as active users.

And if Sword/Shield doesn't do these numbers, then Gen 9 will do it for sure.

I don't think so. A lot of people probably bought USM and were S/M owners.

I think it would have done 18-19 millions witout USM.

I'm one of those people and yep, I think you're right

That's true for all games that get GotY, or Ultimate editions though. Even if 4 or 5 million people bought the game twice, a sale is still a sale. 

People need to remember Pokemon is in a very different place now compared to a few years ago. The series has gained a lot attention with Pokemon go and the detective pikachu movie. Which might help boost sales a little bit. I don't think it'll beat gold/silver but diamond/pearl is a very doable. Also adding s/m and us/um together is a bit silly. While I'm sure some people who didn't play the originals bought it I think that game had a lot of people double dipping.