I feel like they are setting up for D/P remakes. They can finish the dex and release a relatively low effort remake to make the fans happy.There's also the fact that the Sinnoh starters themselves still aren't in, as are several notable Gen 4 Pokemon (Staraptor, looking at you), namely the Gen 4 mythicals who are notably excluded from Sword and Shield despite several other mythical Pokemon making it in.
Also someone made a list pointing out this with potential Sinnoh remakes in mind, here's the image:
Basically the color coding for it is:
Green: In SwSh from the start
Orange: In the Isle of Armor or Crown Tundra
Red: Obtainable in DPP in-game (and therefore would likely be available in the Sinnoh remakes)
Brown: Dual-slot DP (they may still be in the Sinnoh remakes in a form)
Blue: Transferable in SwSh
White: Cut, not in Sinnoh or Galar Dex
If you add in potential Sinnoh remakes into Gen 8 as the next Gen 8 games, everything from Gens 1-4 is covered except the Johto starters and Deoxys: however, the former could easily be obtainable in a Sinnoh remake post-game like ORAS and HGSS did with other starters, same with the Unova and Kalos ones, and there's a Meteorite in Veilstone for Deoxys to forme change so I imagine Deoxys will be available in some capacity in a Sinnoh remake, presumably via event. Meloetta and Hoopa are the only other missing mythicals, which is the only question mark about that.
The rest are all Gen 5 onwards normal Pokemon, and obviously they weren't in DPP since they didn't exist yet back then, but could very well be added as available Pokemon in Sinnoh remakes. Like I could easily see the Flabebe line showing up in Floaroma for example, and stuff like Alomomola and Bruxish showing up in one of the Water routes or something. Each gen they did remakes every Pokemon that was available at the time was basically covered between them and the original paired release (RS, FRLG, and E made all 386 Pokemon at the time available in Gen 3 between each other for example, same with DPP and HGSS, and later XY and ORAS). Meaning that if/when the Sinnoh remakes happen (and I would love to revisit Sinnoh again, for one), they would essentially cover the rest of the missing Pokemon that are still not in as of Crown Tundra. Just a thought.
Then again this is Gamefreak so it wouldn't surprise me if they did exactly not that.