It's definitely to hurt the brand. They know long term it benefits them to weaken the brand, regain control of it, and strengthen it again. If somehow that doesn't work, they're STILL strengthening the brands they gain more from like the Avengers, it's win/win for them, if somehow the Xmen became garbage nobody cared about and they also didn't get the movie rights back, it would be a shame for the comics universe but for the bottom line it wouldn't be a bad outcome (not the one they want, however).
What's the time frame for a plan like this? Realistically when do we expect the X-men movies to make so little money that Fox is begging to unload the rights back on Disney/Marvel?
They're basically doing it out of spite and taking all the fans who love those characters along for the ride.