RoboVM has been free for libgdx users. Xanarin is disliked by independent developers for its pricing structure. Will RoboVM remain free to use? I hope so
Define "independent developers". Because a lot of them do use Xamarin.
A lot of people are vocal about the price, use it or not; you can't pretend it's not true.
However, I do think their asking price for the edition that lets you use Visual Studio is a bit steep (I also pay for it, but still think it's kind of high).
As the copyright holder RoboVM (or Xamarin) can chose at any time to close source the code and change the license for new versions.