They still had to rig the models, which was apparently more challenging than you'd expect based on what some modders have said, add textures, create movesets, in some cases add additional animations... It's not necessarily free, and if it were they would have just done it rather than deal with the expected backlash.
And they're adding bump mapping and higher resolution textures to pretty much all of the Pokemon models in the game. They might have new animations too. You seriously seem as if you believe that GF are just doing this to spite fans.
And source on the BDSP claim, please.
I'm not saying hard work doesn't go into models and rigging or that it doesn't take time. But in the 2019 Famitsu interview regarding the cut Pokemon, Masuda said they cut Pokemon in order to prioritize working on higher quality games (basically going from working on 3DS games to the much higher quality Switch). Ohmori said, "Now that we are moving to the Nintendo Switch and
remaking the models from scratch, we had to make some kind of choice." And I'm quoting that from a NintendoLife article which specifically wanted a human to retranslate that Famitsu article to make sure it was actually accurate. If they were remaking the models from scratch, I mean, it would be a boneheaded decision, but that
would be a reason to cut the count back. But then the games came out, and it was confirmed that the previous Gen models were the same and animations were also recycled. So if the reason given for why they were cutting a ton of Pokemon was a lie, then what WAS the reason? That's why this decision has been baffling to this day, not because I somehow think Game Freak wants to spite fans.
What?
If we relate this back to the question that started this, what would be the excuse if this happened again? We know they reuse the models, which is the smart thing to do. And by now, there's apparently only 60 to 63 Pokemon that haven't been in SwSh, BDSP, or Legends Arceus (not including other forms, mainly or exclusively Alolan), which is far fewer than I thought. And of those remaining, we currently don't know which ones could be added to Scarlet and Violet. If the work has been done, especially recently within Gen 8, what would be the excuse to not include every Pokemon in a single game? (Again, not necessarily catchable in the region, but available with transfers.)
Also, while my point still stands about game file sizes and DLC/updates/patches not being a factor for not including Pokemon, you were right to question that bit about BDSP. In short, I saw that presented as fact, when it was actually a desperate attempt for fans to rationalize why those physical games were shipped in an unfinished state that required a large day 1 patch. (I'll edit my post, in case anyone else happens to read it.) The longer explanation that I pieced together is that when the games released, they needed a very large day 1 patch (in comparison to the post-patch file size). It was officially reported about a week prior to release that the patch was about 3 GB. After the first patch, the game file size was 6.7 GB. If you don't patch them, the games on the cartridges don't have the opening, title, and ending credits videos, the post game, or the BDSP soundtrack (instead it has remade DS versions of the songs or something). So people wanted to know why the games were shipped like that. And it seems the most plausible theory was that if "3 GB" was the literal patch size, then "3.7 GB" was on the cartridge. Why would that be important? Possibly to put this on a cheaper 4 GB cart rather than an 8 GB cart. Companies do like to cut costs, so that seemed more plausible than "lol The games weren't finished in time." And you'd think that if anyone wanted to prove this, the cartridge size would be known by now, but I could not find confirmation of it being a 4 or even 8 GB cartridge. So... this is also still a mystery for why they were shipped in an incomplete state.