Recall that DQXI released on a Nintendo system, the 3DS, and on the PS4 at the same time. Also, Dragon Quest has been solely developed for Nintendo systems for most of its main entries: 8 developed for Nintendo only, 2 developed for Playstation only, and DQ11 developed for both Nintendo and Playstation at the same time. Lastly, the success of the Switch cannot be ignored. It is the only console Japan will have with a good install base until the Switch's successor is released, and Yuji Hori is most definitely not going to ignore Japan in deciding which platforms to bring DQ12 to.

Given all of this, there is a decent chance DQ12 will be a Switch exclusive. However, there is also a good chance it will be Switch/Playstation multiplat. There is a 0.0% chance that it will be a Playstation exclusive.

I bought it on PS4 and loved it. I would have bought it on the Switch but it became one of those Nintendo games that don't drop in price. Got the Definitive on PS4 for $20. It was my first Dragon Quest game so I wasn't willing to drop full price on it. Once I played it I was hooked, so I would be more likely to pay closer to full price on future installments.

DQ XIS on Switch sold 660k physical copies in Japan. So likely over 800k in Japan with digital and over 1mil WW. Out of these 660k copies 100k are the new price edition which sells like an evergreen in Japan. On the PS4 the S version didn't sell much, despite the lower price than the original release and quickly vanished from the Famitsu charts. Considering Japan still is the main market for Dragon Quest, Square Enix would have to be stupid not to launch the game front and foremost on the Switch.

Bold: both versions launched the same day in Japan.

Why would they ignore the biggest platform in Japan (and, well, everywhere).

My mistake, the later date was for outside of Japan only.  I'll edit my post.

You are also forgetting that Dragon Quest XI launched on 3DS as well. A lot of the initial sales in Japan were from the 3DS. Extrapolating Switch Dragon Quest XI sales: It was at 4M as of November 2018 before the Switch version released and after the international release. By December 2019 it was at 5.5M, which is after the Switch version released. By September 2020, before the release of XI S on other consoles, it was at 6M. According to Famitsu the NS version sold about 550k copies before the "new price" version was released where it sold another 150k or so. Based on the numbers given the Switch probably shipped over 1M units. Especially when you factor in digital as well as international.

Anyway, Dragon Quest IX was on the DS. Dragon Quest X was on the Wii and 3DS. Dragon Quest XI was on the 3DS. Square Enix doesn't really care about power for the Dragon Quest series. It's honestly all about the Japanese audience.  

I can totally see Square Enix taking Sony's money for timed exclusivity, attempt to power through the loss of Switch sales by making it PS4+PS5+PC and then 2-3 years after its initial release, in a Dragon Quest focused stream, announce a port to Nintendo's next platform in addition to announcing Dragon Quest XIII...which Sony will also buy timed exclusivity for.

That's the typical scenario for Final Fantasy. We don't know when's the remake supposed to land on other platforms. I feel like FF7R has become irrelevant after all this time so it won't make any difference if it released anytime soon (or worse, later) on other platforms. Sony's deals of timed exclusivity are really threatening. And i am thninking this way considering the amount of future projects of SqaureEnix that are being locked by timed exclusivity and especially that 2 years deal which feels like scandalous.

Hopefully, DQXII won't be treated that way, and i feel like Nintendo will put some pressure to ensure the game to be released on switch, considering how much they cherish the franchise as if it was their own.

Why would Dragon Quest XII not come to the Switch? It's the biggest current gen platform right now, and it's the biggest console in Japan by a wide margain. What could possibly convince Square Enix to not bring it there?