I studied IT since 16 years old, and worked in Software Development for almost 7 years now. And even the me from 10 years ago can confidently tell you that GameFreak's migration policy is not sustainable forever. To me, it is a miracle that they allowed it all the way to Generation 7. In all honesty, I really though they would have done this back in XY when they started constructing the 3D models, but no, they surprised me and far exceeded my expectations when they allowed full migration from non-3D to 3D.
I can't say I fully support their policy on not allowing the National Dex, but rather I fully understand why it is not feasible. I am not saying it is technically impossible, but I am saying that it is unfeasible in a business standpoint.
To break this down, if all Pokemon were to be allowed to migrated for every Pokemon game, GF would have a huge backlog of work to do before they even start developing the new game.
Let's take it that they are able to reuse the 3D models from XY. For each game, they usually reshade the models, they have to come up with new movesets, new Pokedex entry, test that animation model matches the any new moves, give them their Dynamax model and make it they don't screw up the screen, imagine wailord covering the entire screen and you can't see anything, that isn't good.
We have 809 Pokemon now... if it they spend 20 hours on each Pokemon (as believe me, it will make more than 20 hours per Pokemon, this is just a bare minimum estimate. I mean just going their movesets and updating their moves, and getting your boss to review and approve is gonna take like 8 hours already... @_@)
809 x 20 hours, that's already 160000 hours of work before the game starts development at minimum, it usually takes 50% more time, that is uncounted, because people work overtime, and I do not know if they get paid for it.
But, but Nintendo and GameFreak are freaking rich, they can hire more people. There are problems with this too. More people means more clutter, person A submits a work, person B submits a work, person C submits a work, the more separate pieces of work that are produce, it takes more effort to integrate them together and ensure their quality and that they are coherent, it will take more time to review and test the work. Also, are you willing to pay more for National Dex each game?
I can fully understand why GF had to do what they had to do, though it is an unpopular decision.