I think 10 mil will be the minimum but after that it is hard to tell.
As many have stated, the next mainline game lands next year and could stop lets go in its tracks. Lets Go is a remake of yellow, so I can see it selling very well while people wait for the next one. After that though, it depends on how many casuals pick it up. I mean that is clearly the reason they included Pokemon GO mechanics in the game, so who knows.