For what it is worth, I decided not to consider using Unreal or Unity in rviz (when we were porting to ROS 2) partially due to the same license uncertainty. If it’s not licensed under something recognized by the OSI (https://opensource.org/), then it’s not sufficient for us. Now, if it’s optional that would be ok by me, and the ignition-rendering API may allow us to do something like that in the future (where something like Ogre3D or directly using OpenGL is the default, but Unreal is an alternative).
That sounds like they’d be moving towards a solution that is less acceptable for a top-to-bottom OSS stack, for those who care about that. Which was one of my concerns when initially evaluating it, that they were more likely to change their license to be further away from an OSI approved license rather to one that is approved. So you’d then be locked in to some degree into a proprietary solution.