So the region that restricts immigration of Pokémon from other regions is based on England... too soon.
During the Treehouse Live on day 1 of E3, it was announced that the only Pokémon who can be imported into Sword and Shield via Pokémon Home (once it's live) will be Pokémon in the Galar regional Pokédex. This has upset a great many people and was widely regarded as a bad move. Series producer Junichi Masuda stated that the change was made due to the difficulty of modeling 809+x Pokémon in HD without delaying the game. Seeing as we've been promised a "main series Pokémon game" for Switch starting immediately upon the reveal of Let's Go last year, and Pokémon's transmedia property status, this is fair enough.
A hope I've heard on the E3 floor and expressed in social media / the NWR Discord is that the missing Pokémon could be added in an update. But...
Pokémon has had multiple breaks in continuity over the years, and the solution to this problem has always been to buy another game. Or two. Or three.
The first break occured in 2002 (Japan), with the release of Ruby and Sapphire. After throwing off the shackles of the original Game Boy, the inner stat system was completely overhauled. Originally, four hidden values determined a Pokémon's HP variance, shiny status, and even their gender. (Fun fact: Unless the species was only female, second generation female Pokémon can never have the maximum Attack for their species. So Gen 2 is sexist, and also bad.) Ruby and Sapphire blew this out to six different stat variables, along with moving shiny status/gender/the spots of a Pokémon to their own variable. And for over a year, it could only have 202 Pokémon. Colosseum added approximately 50 more depending on the region, and Fire Red and Leaf Green added the remainder. The first two generations couldn't move forward until the Virtual Console releases of 2016-2018.
Technology marches on, and we come to Pokémon Omega Ruby and Alpha Sapphire for 3DS five years ago. Following the introduction of Mega Pokémon in X and Y, OR/AS nearly doubled the list of Megas. And X & Y had glitches that were patched, so the community at large felt a patch to keep compatibility between the two sets for battling was inevitable. But there was no patch. To this day, you can't use say, Mega Altaria in a battle if the opponent is playing X/Y, because the game does not recognize it.
The situation was exacerbated with the Sun and Moon games. The original games didn't have a National Pokédex in them to begin with; this function was offloaded to Pokémon Bank when it updated. The Ultra games did something never seen before: brand new Pokémon in the middle of a generation with the Ultra Beasts (as well as the ability for Ultra players to catch the Totem boss Pokémon). For the second time, fans pleaded with GameFreak to update Sun and Moon. And for the second time, they were rebuffed. The National 'dex also remained in Bank.
We don't really know what Sword and Shield's file size will be, but a freshly downloaded Let's Go Eevee is 4.1GB as it sits on my Switch. Let's Go has data for roughly 200 Pokémon, including animations and various camera angles. With the number of known Pokémon in the game already courtesy of Joe Merrick at Serebii.net, we can assume that Sword and Shield will be larger. Depending on how many Pokémon get added or spotted going forward, they may have to add full models for upwards of 600 Pokémon, which could put the game into that "must have microSD card to play" range even if you ignore the Pokémon that have multiple forms.
We still can dream. Sword and Shield come out in November, and we may get more details on their plans for going beyond Galar even after launch. But as someone who has a "living" Pokédex (one of every Pokémon) that I've built up since 2003, I'm fully expecting that my first Sapphire starter Blaziken will never get to see the Galar region until 2020 at the earliest. More likely, she'll spend her days in Home. And that's incredibly deflating.