Nobody can take away your rights if the contract you sign with the engine developer doesn't allow it, as people have been pointing out is the case for other engines. IIRC Unreal's contract gives you access to a particular version of the engine in perpetuity, with source and the permission to make your own additions/changes to that source. It doesn't guarantee updates, but neither does the MIT license.
Apparently this is exactly what is happening with Unity[1], where the TOS previously said that you can continue using an old version, but now they are walking this back and trying to apply fees retroactively. Whether this is legal or not might be debatable, but unless users mount a legal challenge they are probably stuck paying or finding another engine.
