I think I just blew my own mind and feel dumb for not having predicted this xD...
At this point it should be really obvious to everyone that the Alola forms (or rather which PKMN got them and what types they had added) were made specifically based on what the first generation of PKMN isolated is lacking in type diversity (in the context of the 3 new types since then, as well as overall).
I realized that early on (they weren't based on popularity and they definitely weren't random or shared any other traits, besides complementing the original Pokedex really well with every single addition), but didn't understand why they would ever do that, and in hindsight, it was possible to deduce merely from the Alola-form selection that GF had planned a Kanto adventure that pretended that even its neighbour-region of Johto doesn't exist yet, which essentially would have told you a lot about the kind of games Let's Go would eventually have turned out to be.
For example, the most prominent type the Alolaforms introduce to the Kantodex, is the Dark type. Those added Dark types in forms of Rattata, Grimer, and Meowth, all of them full Alola-forms (in contrast to Alola-evolutions, which I argue should be considered a separate category more akin to location-based evolutions, rather than regional variants per-se), giving the Kantodex 3 basic and full evolutionary families of the Dark type to choose from rather than none.
And you could tell from that, that it was going to exclude anything post-generation-1 by the simple fact that as soon as generation 2 came about, this issue was actually already canonically addressed, when GSC gave Kanto some Dark types (that were even exclusively native to it at that point, despite the reigon being locked into the post-game, making this seem like a "fix"...which makes Let's Gos ignorance of it kind of ironic) with Murkrow and the Houndour line, pretending like they were always there and we didn't encounter them simply due to the absence of a night-time.
Same with Vulpix and Sandshrew offering Ice options to the dex that otherwise has basically no "typical" Ice type to even show the type off in a cliche fashion.
Same with viging 2 Steel lines in Diglett and Sandshrew, adding to the otherwise only Magnemite line.
(The most recent type, Fairy, only got one with Ninetales, due to it having retconned a healthy number of PKMN across all generations, giving Kanto 3 Fairies already)
They also treated the 2 rarest types in gen-1 (Ghost and Dragon) equally, by keeping them quite rare, and merely adding a single alternative, that is an Alolan evolution with the mentioned type as a secondary evolution-gained type (Marowak and Exeggutor of course).
I'm not sure if I'm getting to some bigger point, but I guess #1 is for the people who have been wondering why USUM didn't add more Alolaforms or why Alolaforms were exclusively generation-I, was because they were conceived with a Let's Go plan in mind, and more importantly, #2 implying that Let's Go were already planned as far back as the days when Sun and Moon were still in development!