It'll sell 8 figures. Its value lies not in its sales potential (else they wouldn't be launching it in January dead air) but rather in what its development taught Game Freak to take into further entries into the series canon. And rather being a spin-off, it's tied more closely to he original ethos of Red and Green (explore a region, filling out the pokédex) than any core game since, which subtly changed the stories they told
Even Game freak themselves acknowledged that, when they sub-contracted ILCA to make BDSP while they worked on Legends. It was a required dev process, regardless on if they launched a game at the end or not - after starting to take free-camera and open world-ish elements to SS and furthering that with its DLC, they needed to make a full title to the same ethos, to help break their reliance on fixed camera routes. I expect Gen IX to be more of the same of Hisui's make up (if rumours are true, Prof. Laverton hints at the next region by mentioning he's away travelling again, at the end of Legends). Anyone trying to tell you otherwise is lying to you, probably as they don't look at the overall picture, but rather release-by-release in a vacuum.
