Personally, I don't think Legends will be a one-off thing, nor do I think it will permanently replace traditional mainline games going forward. In other words, I don't think Sword and Shield will be the last traditional mainline entry, and I don't think Legends: Arceus will be the only game of its kind.
That being said, it's impossible to predict what will happen in the future, as GF has a strong belief in being unpredictable and throwing surprise bones on a whim depending on whatever project they feel like. But that being said, while it's true that Legends: Arceus is a mainline title, and they've made it emphatically clear that it is one, as even on Twitter they grouped it alongside the other mainline games in the series thus making it clear it's a core title, I do not think it will be indicative of every mainline title going forward. Rather, if we want an idea of what the mainline games could be like going forward, it's better to look at Gen 8 as a whole, collectively speaking, or better yet, the collection of Switch mainline games that have been released thus far including Let's Go Pikachu and Eevee, which is also mainline, rather than any individual game as an indicator of what mainline games will be like moving forward.
What I'm trying to say is that the Switch games collectively speaking are indicative of a very different direction from the Pokemon games on previous platforms. There is no case where every single mainline game on the platform is the premier, traditional mainline experience that is intended as the main game for the year, the main competitive hub, and whatnot. This isn't like the GB games where RB, Yellow, GS, and Crystal were all following the same formula, or with the DS games with DP, Platinum, HGSS all being the "main" Pokemon game in succession one after the other, with each one replacing the previous as the main VGC hub and features carry over from one game to the other, likewise with BW1 and BW2. Or the 3DS like XY and ORAS, where ORAS replaced XY as the main competitive hub, and how USUM replaced SM and vice versa. In each of those cases, with each game within the generation, features carried over from one game to the next, and they were all part of a collective "generation" sharing features.
With the Switch games, however, every game is vastly different. We have four different mainline titles on the platform, but overlap is minimal at best, and the only two games that could be argued to be stylistically similar and have overlap of some features in the vein of say, RSE and FRLG, DPP and HGSS, or XY and ORAS, or BW and BW2, or SM and USUM, or GS and Crystal, or RB and Yellow, and whatnot, are Sword and Shield and Legends: Arceus, but even then SwSh has multiplayer, Legends does not. Going down the list, we have all of the following:
- Let's Go Pikachu and Eevee, a Pokemon GO oriented style game with a lot of distinct features unique to itself. Note that not much of it carried over to the traditional mainline game that followed.
- Sword and Shield, the first and so far only "traditional" mainline game on the Switch, even if it clearly was experimenting with ideas for Legends. It follows traditional Pokemon gameplay that has persisted on the vast majority of mainline titles and is the main competitive hub for VGC and Battle Stadium, and uniquely unlike with past games, does not get replaced by the remakes/upper versions of the generation as the competitive hub for the generation: for Gen 8 thus far, Sword and Shield remains the main competitive hub.
- Brilliant Diamond and Shining Pearl, a minor-scope remake of Diamond and Pearl that has no connection to any of the other Switch games thus far.
- Legends: Arceus, an experimental mainline title that deviates from traditional gameplay elements and does a lot of bold experimentation for the franchise's gameplay and structure, all the while still staying true to what makes mainline Pokemon, well Pokemon at its core.
Now I'm not saying we will get exact games like this each and every generation, but what I am saying is that future generations of Pokemon may have a wider variety of mainline titles that each do their own thing, and each generation I imagine will have at least one "traditional" mainline title that follows traditional gameplay, with the traditional 8 Gyms, League, and Champion structure, has a battle facility, and multiplayer/VGC, and serves as the main competitive/multiplayer hub of the generation. After all, that will definitely be necessary each generation: competitive Pokemon isn't going away anytime soon. Now granted, the base Gen 9 games will probably adopt some elements from PLA, just as Sword and Shield still took some elements from LGPE despite returning to a traditional mainline experience, and some of the changes PLA made to the gameplay will probably carry over, but the base Gen 9 games will probably still be more traditional like Sword and Shield, and will have multiplayer, battle facilities, VGC, and whatnot, and then we will see some experimental titles like Legends: Arceus and Let's Go both were: perhaps toying with new ideas and deviating from tradition and serving as stronger single player experiences while the base games of the generation remain the multiplayer/competitive format for the entire generation. What I'm saying is that not every single game will be Legends: Arceus to the exact T, but experimental titles that serve as the second/third games of a generation will probably happen while the base games of the gen could be more of a standard Pokemon experience.
I also don't think Legends will be one-off, and I do think some aspects may carry over to the base Gen 9 games, but as a whole Legends could create its own subseries within the core series, same with Let's Go. We could see a Let's Go Johto game later on down the line, but we could also see Legends style games for Unova, Kalos, and Alola showing up as time goes on, with the Legends-unique mechanics showing up in experimental titles. I don't think any singular title is indicative of what future mainline games will be like: if anything, this suggests that mainline games will individually take a variety of different directions: we could have GO-like games like LGPE, traditional games like Sword and Shield, and the Legends: Arceus styled games and whatnot all as individual entries in a generation.