Any new Sinnohan evolutions won’t be in a state of regional limbo. They’ll be from Sinnoh.Mr. Rime can be in the Pokédex if they need to include him. But no Sinnohan forms for Pokémon obtainable in Diamond & Pearl (like Sudowoodo. You forget that it evolved from Bonsly, a Sinnoh Pokémon), nor Pokémon outside of the Sinnoh Dex. Also, I don't want any Pokémon to be in a state of regional limbo like Meltan, so Calyrex has to be the last Pokémon in the National Dex until Generation IX.
Also, I don’t think GameFreak will let National Dex numbering stand in the way if they want to do something. National Dex has been gone for two generations, I doubt it’s coming back; it’s clearly not something GF care about having any more.