Was it Ruby/Sapphire the last time this happened? You couldn’t transfer RBY/GSC pokes to gen 3 as the system had changed completely underneath (EVs, IVs, abilities etc), so if something wasn’t in the dex, it was out - at least until Emerald brought loads back in iirc.
Would’t be surprised if there’s a third game that brings back the missing mons in the same way - Pokémon Halberd or something
Let’s Go Pikachu/Eevee were pretty well-polished, I thought - kind of a dry run at a full gen 8 without some of the complex stuff like EVs, items and abilities. That game beat the pants off the 3DS games graphically, very nice (not jealous)
That said, you’re right about them rushing games out. Sun and Moon were fast-tracked after Pokémon Go took off, which is why they contain some content planned for Pokémon Z (ie the Zygarde stuff). Now, after Let’s Go and Detective Pikachu, they’ve got to strike while the iron’s hot and get the game out by Christmas (for market reasons). Seems like leaving pokes out is the easiest way to reduce the workload - I’d far rather they get the game right with fewer pokes than have them all in a bad title
They can always add pokes in later on as well, they’ve always released legendaries down the line in each gen and ah christ they could release packs of DLC mons, like the Sinnoh starter set or something ah balls that’s it isn’t it