The Japanese already have Generations Ultimate (or Double Cross in Japan) for quite some time, on 3DS and Switch. And they didn't buy it over MHW (although VGC had Ultimate in both version together at nearly 2M, MHW is a bit over 2M). Which is understandable, as MHW is the next mainline entry, while MonHun Cross (or Generations in the West) was a spin-off and Double Cross (or Generations Ultimate in the West) is only the enhanced version of it. Monster Hunter Cross did actually outsell MHW in Japan, although it is close and with digital very likely MHW is in the lead.
Makes sense. I didn't realize this game was just a 3DS port.