@Northwind I dunno if that's the case as I feel we've yet to see MANY pokemons in the open world sections(which are more "open world style" it sound like than what we traditionally think of as open world... but only the actual release will tell us what it's actually like) so things might still be more on the reasonable end. 

But I DO think that world design played an hand behind the national dex cut, even if an indirect one.  Apparently they'd been looking to cut National Dex back in Gen 7, and thinking about it... it might correspond with the fact that Gen 7 is also the one generations where the world began to be rendered in more "organic" fashion, if it makes sense. 

Even in past 3D games such as X/Y and AS/OR, the world was very "gridlike" outside of some exceptions(such as Lumiose city).  It was also a very simplified kind of 3D for a lot of character/pokemon models when moving around the world.  So chances are, even with the jump to more advanced graphics... the world was still relatively "simple" to model and render. 

With Gen7, though, they made a huge jump not in battle graphics but instead in the visuals of the world.  More naturally proportioned characters(and thus the ability to support more complex movement animations/etc), but also more organic roads/hills/etc with curves and design that actually is very non-gridlike in layout... and likely more complex to model and create.  Even some of the houses feels like they have more detailed interiors than before. 

Considering that's -also- around when they considered cutting the national dex, I suspect that the upgraded worlds layout thus represented more effort to develop than they were used to.  And now Sword/Shield adds "open world"-style regions to the game as you said, which likely represent a lot more effort to make. 

And here's the catch, with effort needed to create thing come another critical ressource that I suspect is the one they sorely lack: time. 

Previously, this might not have been as much of an issues, especially with any other company.  But Game Freaks are not just making games.  They're making -pokemon- games that ALSO must thus tie-in with animes and movies or other events pushed by the wider Pokemon Company's animation/etc studios that drives the general pokemon IPs forward. 

And that's the catch, I suspect, there might have been an era when it was the games that were pushing the anime forward and in fact the anime even kind of existed apart from the game when it introduced events and even locations that had nothing to do with anything that might have happened in games.  But as time went on, you can see there was a clear desires to directly correlate the events and plots of games with those of the anime... and to a degree I thus suspect that it's now the games that are pressured to keep up with the anime rather than the other way around. 

And thus Game Freaks are stuck in the unenviable position of having to follow a pace of development even more demanding than in the past, because of factors they can't exactly control themselves(the animes, which are pushed by different parts of the Pokemon Company)... all while having to do so in a context where more and more is being asked out of their games: more advanced graphics, more developed worlds(to go with graphics), new mechanics(could dynamax have been a creation of the show rather than Game Freaks is a thing I suddenly wonder about) and so on. 

And without even having the freedom of going "Well, we'll take more time this time around" to make sure they can indeed put -everything- they can... because that game has to be released on time so that the anime can release alongside it's designated tie-in game. 

Seen that way... I think it's no wonder that national dex or similar features would end up being cut at some point.