We have to understand too this might be one of the most expensive games ever made. Leaving it just on the PS4/PC may not sit well with shareholders.
Even if the bulk of sales are on the PS4, I'd say between XB1 and potentially an NX platform, Square-Enix could get an extra 2-3 million in sales, so I can see why they wouldn't just want to flush that down the toilet.
If the NX is a portable for example which has a Tegra X1 level processor, it's not impossible a port there would be doable, and that could honestly be the top selling version in Japan. So I can see why Square-Enix wouldn't neccessarily want to throw that away.