The time has never been better for a new “game engine” or 3D app framework. Unity is trying to be everything for everyone and hasn’t done a great job of taking care of its core audience: indie game developers.
Unity is the de facto choice for XR developers but it’s a poor application framework. Something like StereoKit that caters to non-game devs and can provide rendering, a flexible interaction model, UI, and a good multi-user app model could really steal Unity’s thunder for multi-platform development.