I think a prediction can probably be made based on a few factors:
As can be seen here
>>39060373, the last two games where vaguely similar things to the whole 'Dexit' debacle happened, Ruby/Sapphire and Black/White, both sold comparatively poorly, being the two lowest selling sets of generation starting games so far. This is especially noticeable in Black/White's case, where they came immediately after the best selling games since Pokémania ended. The through line here is obviously that there was some sort of catch
pun intended to "catching 'em all", which is something this fanbase has historically hated, and considering Sword/Shield seem to be receiving possibly the biggest backlash yet for doing something like this, it's probably safe to say that sales will be negatively impacted by this (along with all of the other negativity that's started to crop up).
Also worth pointing out is that Pokémon hasn't been the best selling game on the platform its been on in at least a decade, and there's one series that has consistently outsold it in that time period; Mario Kart. Mario Kart 8 Deluxe is the best selling Switch game right now at 16.69m units, and Pokémon usually trails those games by anything from 2m to a whopping 8m units, so it's unlikely that Sword/Shield will hit the 16m unit mark that XY and SM did.
Finally, there's the obvious $60 price tag, double the price of previous games, which obviously won't put everyone off but will probably still cause a bit of a drop off, especially combined with the aforementioned backlash. I imagine quite a few people will be convinced to save their money.
Based on these factors, I think ~14m lifetime is probably a pretty safe bet. Could go a lot lower, but probably won't go much higher. Would be a pretty poor showing for mainline Pokémon's hyped up console debut, all things considered.
