I don't know, I'm having a hard time believing Disney would put all that time and money into developing and promoting Rebels (and creating a bunch of new original characters), only to ditch the entire thing after 4 seasons and start all over again with yet another new show.
It makes sense to me from a marketing perspective. The upcoming movies are the core of the strategy, the big deal that everything else revolves around. They want to keep interest in
Star Wars alive prior to the premiere of the new trilogy, but they don't want to spoil anything about the post-ROTJ universe, so they keep the franchise in the public eye with a show that builds on the original-trilogy setting. Then, once the new movie's actually out and the sequel-trilogy era is what everyone's talking about, they do a show that capitalizes on that era.
Besides, 3-4 seasons is a pretty typical run for an American animated series these days. You generally don't see a show running for more than 65 episodes. US animation depends heavily on toy licenses, and if the kids already have the toys for your show, they're not buying as many new toys. So you cancel it and start up a new show so you can sell a whole new toy line. This is why long-running kid-show franchises these days tend to reinvent themselves periodically.
Power Rangers (or rather, Japan's
Super Sentai from which it's adapted) changes its title and introduces a whole new set of Rangers, villains, weapons, and robots every single year. The
Ben 10 franchise is 9 years old and is on its fourth series. If
Rebels runs as long as 4 years before giving way to the next iteration, that's actually pretty long by today's standards.