Their blog has a lot of posts on trying to attack Firefox. If it's so simple, why are you not in the bug bounty hall of fame? :)
It is also made in a way that it is optional (never break the web mentality), so what happens in practice is the same as with CORS: allow all, because web devs don't understand what to do, and don't have time to read the RFC.
For example: try getting a web page to run that uses a web assembly binary _and_ an external JS library. Come back after 2 weeks of debugging and let me know what your experience was like, and why you eventually gave up on it.