EXT_mesh_gpu_instancing: used vs required?

[zeux]

The spec for EXT_mesh_gpu_instancing doesn't mention the expected usage pattern: there are some extensions that modify the appearance of the scene in a way that can be insignificant if unsupported -- for example, for a lot of the new material extensions a renderer that doesn't support them will just render a slightly more basic version of the object, so I'd expect most material extensions to be used- but not required-.

Some extensions like Meshopt or Basis allow required- style usage with no fallback if desired, and optionally support a fallback for renderers that don't support the extension (with associated penalties on load time / size / memory impact) in which case the extension would be used- but not required-.

It's not clear to me what the expectations are for EXT_mesh_gpu_instancing. If extension is not supported, the rendered result will omit all instanced meshes and instead render a single instance of the basic mesh with a wrong transform. There isn't really a useful way that I can see (please correct me otherwise) to specify a fallback.

However, the instanced example here https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/SimpleInstancing doesn't list the extension as required. Any renderer or tool that doesn't support the extension will render a single cube at origin.

Should the extension spec recommend that scenes that use this extension put it in requiredExtensions? Should the model above be updated accordingly? Or am I missing some subtlety here?