Always important for longevity
Pokemon games often live or die by their endgame activities. Silver and Gold are still celebrated today for allowing players to return to the original first generation Kanto region, for instance.
While Pokemon Let’s Go‘s legacy is yet to be seen, according to director Junichi Masuda, the two games will have a “difficult” endgame. Speaking to Polygon, Masuda stated, “A good portion of the more difficult stuff is in the post-game. There’s ways to strengthen your Pokemon, and the reason you wanna do that is, you take on these master trainers, these really powerful trainers that you encounter after the main story.”
This is presumably after the Elite Four final challenge as Masuda specifically mentions “after the main story” as a qualifier. As we can see from the latest trailer the three legendary birds and Mewtwo are still in, so with these “master trainers” there’s a bit more to do. Whether everything from the first generation is still in or not is something we’ll have to find out on November 16.