Pokémon Sword & Shield might only contain a limited number of Pokémon, but it has been confirmed that the Pokémon that don't make it into the Galar region will be included in future games in the series.
The Pokémon series has always included every Pokémon from the previous entries in the series, which has made the goal of completing the Pokédex increasingly difficult with each new generation. Pokémon Sword & Shield will be the first game in the series to abandon the National Dex, which means that it will only be possible to capture & trade Pokémon that appear in the Galar region. There has been a significant backlash against the developers of Pokémon Sword & Shield for this reason, even though it has yet to be revealed which Pokémon are being cut and how many will make it into the games.
The Pokémon who won't appear in Pokémon Sword & Shield are still eligible to appear in later games. Junichi Masuda recently spoke to Game Informer and he confirmed that the cut Pokémon will definitely return in future Pokémon games.
"You can look forward to seeing Pokémon that don’t appear in these games appearing in different regions in future games. I think Pokémon Home, for a lot of players, will serve as a launching pad to gather them all there and then embark on future adventures."
One of the common comments regarding the absence of the National Dex is that Game Freak should add the cut Pokémon gradually into the game through updates, which would become a live service for Pokémon Sword & Shield. It feels as if Game Freak will eventually release a Pokémon game during this generation that includes all of the Pokémon from the series, but they will need to be introduced gradually through the framework of the other games first.
The absence of certain Pokémon from Pokémon Sword & Shield was always going to cause a backlash, as every Pokémon has its fans, even the less popular ones, such as Dunsparce and Vanilluxe. The fans of the cut Pokémon won't be separated from them forever and all of the Pokémon can be united under a single Pokédex again at some point in the future.
Pokémon Sword & Shield will be released for the Nintendo Switch on November 15, 2019.
Source: GameInformer