For the same reason people buy smartphones instead of phones and cameras. It's more practical. In Pokémon's case, it's just sick to have 2 regions in the same game, as this means more Pokémon, more challenge, and everything already stated by the OP. Don't tell me you didn't want to get HGSS when it came out, even if not for the reason of there being 2 regions.
More on-topic, I would LOVE to see a game like that *O* Hoenn is my favorite region after Sinnoh, and a game with both Kalos and Hoenn would be..well, pretty darn awesome. Those arguments look fairly valid to me, but being realistic TPCi has never done anything like this so I don't think they would now.
If it was to be released whatsoever, given the nature of the evidence you provide, I don't think it would be released in 2015, or at least not before December. Here's why:
- There was a reference to the Pokémon Dream World (intended or not) in the original RZE. A guy in Devon said that he was working on a device that would allow people to see the dreams of Pokémon (or something very similar, I don't really remember exactly atm) but something like that didn't happen in 2 generations, 7 years; until the release of BW.
- I get the feeling that lately TPCi wants the audience to be hyped for the next Pokémon game. If a game like this was to be released in 2015, at least some information would have been leaked already, with really small clues to a Kalos/Hoenn mix; given that XY were released ~10 months after their announcement, and information had been leaking from even before that iirc. Those 10 months of wait really hyped the games, so much that they were one of the most sold games during their year of release.
So, following this logic, and given that Hoenn alone already creates a lot of hype, I think TPCi would announce this at least a year before its release, possibly in January/February to maximize sales.
- In the Pokémon world, Kalos is far away from all of the other regions, as shown in this map:
Thus, it would make more sense to first link two of those regions instead of Hoenn and Kalos; if they link any at all. I don't consider this a solid reason (especially because the hype argument and this one contradict each other) but I'm just listing it for what it's worth.
As much as I'd like this to be released, TPCi probably won't.
EDIT: What do you mean by your first sentence?