It's not "new and immature." It has a formal specification and is used in production on devices/pcs everywhere. The comment itself was sort of vacuous and nitpicking a single aspect which is something that could be remedied through collaboration.

The set of web safety changes for SPIR-V is kind of new and immature in my opinion. I agree that it's building on a core that is widely deployed in other contexts.

By that standard, everything with respect to a shader ISA for the web is "new and immature." The point is finding a suitable common ground for all vendors to agree on working on. Not everyone going every which way to work on uncoordinated efforts.