Here's why I don't see it happening:
- Time. There is not enough time to release all of these sets. The game economy is delicately balanced right now so that players can spend a little ($50-100 per set) and then over time accumulate all the rest of the cards they're likely to use. Even doing that, most people won't get them all and the vast majority of players won't even get close. Releasing four or more new sets all at once would destroy the game economy and make everyone very angry. If, alternatively, they spaced them out over a year or so, they'd still not really have enough time as it takes a solid three months to build a reasonable collection even after $50. Again, this would make people upset.
- Goals. They've said they want people to be able to take decks they're playing right now and play them in Standard Plus. If you add Kaladesh, it will warp the format to a ridiculous degree. There's no way most of the current decks would survive. So instead of one new format for players to make decks for (new standard rotation), they'd just have two. They'd make the problem worse, not better.
- Design. Ixalan was the first format where the new design team had heavy input. The two standards featuring only their cards have been wildly successful. Everyone loves the balance. Why on earth would anyone take these well-balanced sets and throw in shit like Kaladesh that had a ton of banned cards (and I realize these might be banned again, which is really not ideal either) and mechanics that people generally thought were very unhealthy for the game? That doesn't make sense. The game will be more fun without those sets in it.
- Familiarity. Arena houses many brand new players. So many new sets in a short amount of time would be extremely overwhelming. None of those new players would know what most of the cards do. That's bad.
