E 2021: LGPE are generally considered main series games now (though, as you'll see below, they weren't always marketed as such, and they still bare major differences to the 'average' main series game)
I'm going to write an explanation here for what these games are exactly. Hopefully this will give some clarity. I'm halfway doing it for my own understanding, actually. :P
In short: 'Let's Go, Pikachu!' and 'Let's Go, Eevee!' are NOT main-series Pokemon games, and are spin-offs. Though these games are set in a main-series region and are developed by Game Freak, they should not be held in the same echelon as your X & Y or Sun & Moon. Courtesy of Serebii, we know the following:
- These two new games will only feature the original 151 Pokemon along with some Alolan forms. Real main series games always have continuity with each species that is previously featured. However, the games may still be considered Gen 8, as an unnamed 'special Pokemon' has also been confirmed.
- The two games do not feature the same mechanics as the main-series games. For example, it has been confirmed that wild encounters consist only of the motion-controlled capturing sequence and no actual battles. We don't know the full extent of the changes yet.
- The games have an integrated co-op system, which is a mechanic that has never been used in a main-series game, and deviates considerably from their typical structure.
Junichi Masuda will return to the director's role for this pair of games. He has stated quite explicitly that the target audience for these games is not the same as the main-series games. Sourced from IGN:
“For our first entry on the Nintendo Switch, I really wanted to do something more kind of simplified, something that really anyone could enjoy,” Masuda said.
“I think the reason for this is to get more Switches into the hands of younger kids and more casual players, not just players who are really interested in hardcore games or are really gamers first and foremost.”
Given the game's functionality with Pokemon Go, a lot of people are interpreting this as the Pokemon Company's attempt at mixing the casual, mobile game market with the Switch market. Regardless, you can take Masuda's word for it that you shouldn't expect the same thing from these games as you would from a main-series game. Even calling these a remake of Pokemon Yellow is a bit of a stretch -- they appear quite different.
If you're still not convinced, the Pokemon Company have also confirmed 'an all-new core series Pokémon RPG title' for late 2019 -- again, the implication being that these 2018 games do not belong in the main series. So fret not: these games seem more like something to tide us over until the next main series games release.