So a few assorted thoughts on what this means for the future of the series:
1. They intend for Legends to be a running sub-series of the main games. The fact that they titled it Legends: Arceus implies that they intend to make more games under the Legends brand. Anyone up for Legends: Mew, Legends: Celebi, Legends: Jirachi, or Legends: Keldeo? Legends seems to be intended to be what I like to call a "branch" of the main games, a series of games that are main games in the same genre and with the same core gameplay, but in a different style. For example, the sandbox Mario games for Mario (Super Mario 64, Super Mario Sunshine, Super Mario Odyssey, I also count Super Mario Galaxy but that's somewhat of a hybrid and many people seem to consider it a linear Mario game), they're considered main series Mario games because you run and jump like other Mario games, but instead of running through a linear level to get to the goal, you're wandering around a 3D space to get collectibles. Or the Metroid Prime series for Metroid, which is a Metroidvania like all main series Metroid games, but is a first person 3D game instead of a 2D sidescroller. Now for Pokemon we seem to be getting Legends, which is an open world adventure set in the past instead of a present day linear adventure (Let's Go might also serve the same purpose in being a running series appealing to more casual players if it gets future entries, but more on that in my next point). This is a great way to appeal to differing tastes simultaneously.
2. They're spacing out generations and Gen 9 may be a lot further off than we thought. No one ever thought they would make something this ambitious this soon because Game Freak can only do so much at a time. And it does seem unlikely that after finishing Legends: Arceus they would be able to dive right into another full scale 3D adventure and release Gen 9 in 2023. This tells me that Legends: Arceus could be a "Gen 8.5" of sorts and Gen 9 won't be happening anytime soon. They have a lot more in their toolkit now for spreading out generations so they can spend more time on the big new generations, they have traditional remakes as always, but now they have DLC, they have Let's Go, and they have Legends. I think we'll start to see 5-6 year generations with those kinds of games filling in the gaps. Maybe in 2023/2024 ILCA could give us a Let's Go Johto using BDSP's engine and then Gen 9 happens in 2024/2025. After SwSh turned out to be rushed and garnered complaints, they seem to be slowing things down and finding new ways to get more mileage out of each generation.
3. Game Freak seems to be solving the understaffing problem by recruiting other developers to work on the main games. Another potential issue people have pointed to is that Game Freak is a smaller studio than other developers and they refuse to expand much, and they seem to have been stretched thin lately. So they seem to be solving this by licensing out the remakes to ILCA to alleviate themselves to work on other projects. This is a nice way to get around having to bloat their studio to be too large, and CoD does the same thing to remain yearly while still putting out high end console games (it's developed by three different studios, Infinity Ward, Treyarch, and Sledgehammer Games).
All in all, very encouraging signs for the future of Pokemon, they definitely seem to be listening to the complaints about the games being too rushed.