Speaking about Platinum... I was really disappointed with no Kalos sequel.
However, I think it might actually be better for Pokemon without a Kalos sequel. Maybe because they found that sequels do not really milk that much.
Also, for the 20th anniversary which should be big, if they release a sequel, everyone would think they are milking their old stuff again.
I agree that they should give something completely new for the 20th anniversary.
And if there was one year before the 20th, then maybe Sun Moon wouldn't be so hyped.
If this was the case than GF:
1. Realized too late that sequels didn't sell well but had already finished with XY. XY are games which had left plenty of things open seemingly on the intent they'd be answered or expanded upon in a sequel. But if they decided that since remakes/sequels don't sell well so decided to can the one for XY they're leaving Kalos unfinished. This decision is something you keep in mind for the next gen not just decide to do in a middle of one.
2. They have terrible timing if they didn't allot themselves enough time to release an XY sequel just before the 20th anniversary which is when they plan on releasing a new gen.