Pokemon remakes tend to do around 3 million in Japan, that said, leaf green and fire red are the lowest selling remakes, and pikachu edition was on the low side for an enhanced release. I think it will probably do a low 3, high 2 million between the two editions.
And those things would make Japanese players want the game more than usual why?
There is no nostalgia for gen 1 here, to be nostalgic of something you have to know about it then it has to leave your attention. Go while is a thing here, and people still play it, it is much bigger and powerful outside of Japan.
I think the group of people who really don't like it is very small, compared to the total player base. A lot of fans who buy all mainline games will buy this game too, in the end.
I think kids will like it. I think fans will like it. So the game will have the potential of any other game.
However, the connectivity with Pokémon Go, the Gen I nostalgia boost, and the selling power of known faces Pikachu (marketed as the cool guy) and Eevee (marketed as the cute girl) are all factors these games benefit of, which other Pokémon games don't.
The only thing I can think of that limits sales is the one Switch per household approach. With multiple Nintendo Accounts on one system and co-op play incorporated, families will only need multiple copies where people want to play separately at the same time. However, as proven many times in the past, Pokémon games have to power to sell multiple copies to the same consumer (whether that's an individual or a family doesn't really matter in this case). That may have an influence on when people will buy a second copy, but not really if.
So my guess is that the 'casualisation' of this game only increases its selling power and I wouldn't be surprised to see a higher lifetime sales number than the 3DS games in the end. I'm most worried in regard to its selling power about the next mainline Pokémon game next year, which is a problem Diamond/Pearl and Black/White didn't have to face.
People should not forget that unlike the West, Japan is actually quite familiar with Pokémon beyond gen 1. Pikachu and Eevee are of course even in Japan probably some of the best known Pokémon, however they are cute-kei, not cool-kei which is a huge selling factor for the games. Obviously we don't know how closely the story will stick to red and green, but if it does, I imagine it might be a bit boring for 10 year old boys in Japan.
Nintendo is in an absolute ridiculous position with the Switch right now just counting their own potential releases. They have so much for 2018 already and have plenty of plausible games for 2019, they can really push the system. Should be interesting.