That's assuming that Switch owners who are Final Fantasy fans never played it on PS2, PS3, Vita, PS4 or Steam.
It's about the odds of double dipping. I own it on PS2, PS3, and PS4. The people who are targeted by these releases are people who missed it completely or big time fans of the series. Considering backlog culture, I wouldn't be surprised if people own it on PC/PS4 but buy it again on Switch to have it on a handheld.
I don't think there's a particularly large overlap between Switch owners and PS3/Vita/PS4 owners, let alone the tiny speck that is PC gaming in Japan.
I'd say 75k is the ceiling for the initial week of Days Gone, likely closer to 50k.
There's definitely some overlap. Final Fantasy has a very targeted market and that market is most likely to own a Playstation platform. It's about convincing those who haven't played a mainline FF in a while to give it a shot.