webGL exports an entire ‘app’ including the runtime, and ‘asset bundles’ which are unity-specific assets (no code included)
What I’d like is a way to export (something like a .unitypackage) objects that can be loaded via ThreeJS, BabylonJS etc. There are 3rd party Git projects which attempt this. Having official Unity support would help promote using Unity to author them, rather than jumping ship to PlayCanvas or some other platform.
What’s your expectation? You may get the meshes and materials across to another engine but that’s about it. In that sense you could as well add those assets to the target engine directly.
Also, consider that this won’t be in the interest of Unity to support export to game engines but rather improving Unity itself to match and exceed contender’s features and services. 