For people desperate for even a few more Pokemon this gen, I can say with high certainty that this demo probably lacks the files for future event/mythical/DLC Pokemon. Every Gen since 4 has included three Pokemon that weren't available in the original release, and every Gen since 1 has had one or more. The reason past games included their files anyway was because it was impossible/difficult to just patch in significant new assets, making it much easier to just develop them in advance and the release a code that unlocks that content. In some ways, this is still easier, so why wouldn't those mons being in this Pokedex dump?
Simple, game development cycles have changed since then. These days, it's common practice to develop future DLC in the final weeks of the main game's development, when all of the assets are already finished and all that's left is to bugtest and tweak minor parts of the game. Then, they're introduced into the game once they're released and in the weeks to follow. Given this, I wouldn't be surprised if Gamefreak is modeling and coding future mythical Pokemon right now, and the reason they didn't make it into the demo build is because they're some of the last things they'll add, and this demo took priority over their development. When people datamine the full build, I wouldn't be surprised to see those last three Pokemon, or Gamefreak might take the modern approach and add them through an actual patch.
In any case, Marshadow is almost certainly not the very last Pokemon for SuMo. I don't think there's a whole missing dex with 50 new mons, but I find it more likely that Gamefreak hasn't yet coded in their future events/DLC than the scenario where a) if Marshadow is the only mythical Pokemon this gen, then there's no in-game final legendary beyond the cover legends (hasn't happened since Gen 2) and b) Gamefreak shoots themselves in the foot by not keeping the door open for new Pokemon to be introduced in the time between games, which keeps their brand relevant, which is the entire reason mythical Pokemon exist in the first place. There are probably exactly three mythical Pokemon in each modern generation because some focus group has calculated the ideal time period between releasing new Pokemon for people to keep interest, and it's x/3, where x is the time between releases. It's a matter of logic and business sense.
Sorry if this has already been mentioned, some of the responses I've seen in the past few pages indicate that a lot of people believe the contrary.