They prioritised making a big console(read: PS4) game which meant they released a 3DS game months after its successor launched. The Switch version was an afterthought, enough so that it's coming 2 years late in Japan.
If DQXI had taken off the way they wanted the Switch wouldn't be getting all these additions.
Yeah, thats true. Maybe its my english, but i read this as they didnt think that the Nintendo fans would care, so i was wondering what they wouldnt care about, and what the huge mistake was in regards to this. Reading your post again, i guess you mean that they didnt think that much about the sales a Switch version could bring in?
The developement of DQXI started in 2013 however, and at that point, the Switch likely wasnt even known at all, so it had to be added later on. That they even announced DQXI for Switch that early on shows that they were commited (to some degree at least) very early on to the platform. And since they made a 3DS version too, i dont think they took the Nintendo fans for granted.
I think the Switch version would have gotten these additions regardless. Maybe not every single little tidbit, but at least the main things. If they're going to release the same game that much later at full price, they must offer something compelling to maximize sales (and they've done a great job at that :)). Getting backup from Nintendo could also be an insentive to make more changes.