I think that, if asked, the people at Game Freak and TPC would not say that dex number is the only determining factor of whether or not something is a new Pokemon, and that they take drastic changes such as Alolan forms into consideration. Clearly they have never come out and said that, and I'd argue that they haven't needed to because they've never done something this significant to existing Pokemon. I think that "the dex is the way it is and will always be that way, period" argument is incredibly flimsy when new features like this emerge, so until they explicitly say otherwise, I'm on the "Alolans are new Pokemon" train and will continue to point out how weirdly obtuse the argument against that is.
They could have easily given the Alolan variants their own dex numbers (and some their own name like Snowshrew instead of Sandshrew when it's no longer a ground type) but they didn't. That's more points in favor of GF not considering them new Pokemon.