Candies are pretty much a direct translation of Go’s power up mechanic so I don’t know how that will be confusing. And that’s not an entirely accurate description of CP as despite the name it’s not just the equivalent of Att/Spatt. It’s actually a total of three hidden stats that equate to HP (Stamina), Att/SpAtt (Attack) and Def/SpDef (Defense); it’s arguably more complicated than regular Pokémon stats since two Pokémon with the same IVs/EVs/nature will always have the same stats but two Pokémon with the same CP might be totally different. And as I said, IVs exist in Go, albeit an adjusted version (only for the three aforementioned stats and they only go up to 15). Level is also hidden in Go and has to be inferred visually. The whole system is pretty opaque, you can read more about it here:
https://pokemongo.gamepress.gg/pokemon-stats
If the us long-time players who got into Pokémon as kids could handle natures and held items when they showed up in RSE (yes held items technically existed in GSC but they didn’t remotely resemble how they are now) I’m sure Go players will be able to handle it too considering the age range of Go players extends a lot higher. And remember that Go to Gen 8 won’t be as many Pokémon added as it sounds because Go already has additional gens available that aren’t in PLGO.
I really think people are underestimating how hard it will be for Go players to make the transition. Pokémon is really not THAT complex especially if you don’t care about the competitive scene. The biggest adjustment will be the focus on PvE vs PvP. I also think a lot of people drawing this dramatic chasm between Go and the main games probably haven’t played Go since near its release and don’t realize how much it’s evolved in the direction of the core series.