Pokemon Z would not have saved Kalos at all.
In most cases the third versions expand the dex and give focus to the third legendary, in this case Zygarde. While I do think the plot would have been retrofitted to include Zygarde odds are it would have been a similar case to Alola in which Dexio and Girl Dexio are involved with the Zygarde situation and possibly have doggarde travel around with them. That said odds are Lysandre's goal would still be the same, his admins would still be bland as ever, and Kalos would still be a bland boring region to travel through. I sincerely doubt anything would have been drastically changed in such a way that it would "save" Kalos. Out of anything the inclusion of more pokemon would make it worse since less Kalos mons would shine.
At the end of the day I think Z would have been more like USUM than anything really good.
Honestly I do not think "Z" specifically was going to be a thing. I personally think that the Cells and 10%/Complete forms were thought up after they decided to not have follow-up titles and work on a new gen for the 20th anniversary instead. What little trace data is left in Gen VI shows some interesting things
-There are TWO unused games that, if you hack a Pokemon to be from them, both say they were caught in Kalos
-Both Thousand Waves and Thousand Arrows are in the coding of Gen VI
-There are 2 unused flags for Zygarde forms.
The main thing is Land's Wrath. It has the same power and accuracy as the Thousand moves, but has no additional effect. It's useless. In fact what would be the point of making not one, but TWO unused moves for one Pokemon, with the same stats as Land's Wrath outside of having secondary effects? I believe the original intent of them is that in the pair of Kalos follow-ups, Zygarde was going to get the Kyurem/Necrozma treatment and get two version-exclusive forms. For one Land's Wrath transforms into Thousand Waves, and for the other it transforms into Thousand Arrows.
Whether or not this follow-up pair was a sequel like B2W2 or an "Upper version" split in two like USUM is anyone's guess. But it would almost certainly not only allow the ORAS megas to be used in Kalos, but probably added new ones(The only ones I have any confidence in guessing are probably Slowking and Froslass, similar to how Gardevoir was in XY and Gallade was added in ORAS).
If I'd had to guess HOW early the follow-ups were scrapped, I would say at the latest it was sometime right before the development of the anime's "XYZ" season, due to the Zygarde cells being a major plot point in it.