It's no mystery that Zygarde was destined to have its own story arc, particularly with Squishy's plotline in the anime and 19th movie. However, some have speculated that Pokémon Sun and Moon were rushed out as the last major Pokémon titles on the 3DS before Game Freak moved its development resources over to the Switch. For this to happen, they would need to scrap the development of the Z Version or hypothetical X/Y sequels and move Zygarde's new forms into the new game.

Do you agree or disagree with this theory, and why? Do you think the cores would've been spread throughout Kalos instead?

My assumption is that it went something like this:

"So, guys, we need something to pad the play time of SM a bit."
"Let's make a Pokémon-Snap-minigame-slash-Facebook-parody using the Rotom 'dex."
"Not bad. What about bringing Looker back for a giant fetch-and-fighting quest?"
"I guess. We still need legendary Pokémon. Can we integrate some with the Looker quest?"
"Maybe. You know, I liked the feather sidequest in Assassin's Creed III. Hey, Zygarde never got a proper story arch. What if we split it into a hundred tiny little pieces and throw them at random locations around Alola?"
"I'm on board with that."
"Me too."
"Let's do it."

I'm joking. Considering how short Gen 6 was and how many other plot lines were dropped and how many new things were introduced in 7, I'm guessing they had too much that they wanted to do and figured it would make more sense to squeeze a whole extra generation out of the 3DS instead of just making Pokémon Delta Emerald Z (maybe that's why they called them "z moves?") or whatever before Switching.

Probbly zygaurd was gonna be in z but the ydidnt want to do that, so they made s/m instead to conpensate