XBC2 will sell far better than a hypothetical SMT 5.
SMT has always been a niche even on the PS2 so I have no idea why you are under the impression that SMT 5 will be able to sell 2 million and above on Switch of all places.
By that definition XBC has also been niche since the Wii days. I'm not saying that the SMT franchise will suddenly be un niche, its that the audience will see the appeal of it with a new eye and a huge turnover in sales from a good marketing campaign. A lot of people were saying niche in regards to Crash as well, and are pretty silent now due to various factors
You think a hardcore JRPG of a less popular franchise will easily "destroy" by far the highest selling launch of any Atlus game in history? Good luck with that.
Well let look at the factors.
1. Its on a platform its home country takes to naturally
2. Its a franchise that has endured for generations
3. The fundamentals are strong
In all 3 of these cases, SMT Switch has the advantage on Persona 5. Literally the only thing P5 has in its selling potential favor is the dating sim aspect of the presentation that appeals to otaku.
You say SMT is a less popular franchise and of course it is...but maybe you have forgotten that before P5, Persona itself was not popular either outside of P4 Golden's rerelease and even that was just relatively speaking to the rest of the entrees. I would never discount the possibility of SMT5 pulling a P4 Golden or a P5 with its new advantages