I don't mean to rant, but I think people who complain about the lack of a National Dex don't understand the situation Game Freak is in.
One of the most common complaints I hear is, "They're so greedy and lazy! Just hire more people!" Well, contracting and hiring new employees is not a simple task. Even with large corporations, you typically only hire a few people at a time, not all at once. Then there's a bunch of human resources management involved with interviews, and you need to give a training period to new staff, all of which takes an incredibly long time. Second, just because they have money doesn't mean that new staff is always available. You can't just materialize 50 new 3D animators out of thin air whenever you want. Most of the 3D for the Pokémon themselves are done by Creatures Inc, who are also responsible for the TCG and a bunch of other stuff, and even then they still have to reach out to other companies for help - companies that have schedules of their own.
And even with all of this, I think people still underestimate how much work it is to work with the amount of Pokémon there are. Even if you decide to add only one new animation for each Pokémon, that's already more than 800 animations. You can't make these in a day and call it finished. You have to make sure all of these look good and work with all the attack animations. When you don't, well, remember how some Pokémon would stay perfectly still in their Z-move animations? The amount of staff that would be needed to create all of these assets and have it be perfectly polished would probably result in the equivalent of a management nightmare.
Now I know what people are going to say: "Why won't they just delay the games then?" A delay could have some nasty implications for the franchise that other games might not. For example, the anime. The studio and all the animators involved have their own schedule and other projects they have to work with. With a delay on the games, here's what would happen: With a sudden change of plans, the writers would be forced to lengthen the episode count of the previous series by making boring filler that has little to no substance to the main plot. Not only would that stink for the fans, but it'd likely result in a huge drop in viewers until the new series comes out. That's not even mentioning the TCG stuff, toys, clothes, events, TV ads, etc. It would cause a chain reaction and make everyone's life harder.
There's plenty of design decisions on Game Freak's part to criticise, but I don't really agree with the notion that the series would be better with another studio. The Pokémon series is a scope time bomb that would be difficult to manage by anyone. Even other big scale RPGs, like Dragon Quest, Persona, or Level5's games, still don't have the same scope as Pokémon, and even they have to resort to palette swaps for enemies to reuse animations.
It's hard to admit, but this was bound to happen at some point. No series could get away with a returning roster this big without consequences to the quality of the games. Even Sakurai admitted with Smash Ultimate that he thought he might not be able to do it (also health issues). There are many Pokémon at this point that were created as one-off mechanics or to showcase a new feature, but they still had to be included even though they had no purpose and/or were totally outclassed by other Pokémon. I am an avid defender of the "good design is design with purpose" mentality, and no game mechanic should be included for no good reason. Game Freak decided that this decision would help them improve the game in other ways, so they have at least some of my respect for that. I'm not going to say that it's wrong to be upset that the Pokémon you have been collecting since 2002 might not be available in the new games, but we have to admit that in a way, this might have been holding the series back from trying new things.
And that's just my two cents on the matter. Sorry to ramble on, but I feel very strongly about this.