Actually gRPC is about the same as other HTTP based APIs, it is just a more efficient RPC. All of them are lacking what made the web scalable in the first place.

- HTTP APIs are worse than websites of the 90s. At least a browser could be expected to view a few websites. HTTP APIs require a custom client for each one.

- Document media types, not APIs. This isn't such a novel concept, browsers (fancy HTTP clients) work because HTML is a standard.

- Standards at the application layer, not specifications. You mentioned specs only.

Let me just clarify that RPC is a great fit if you are constrained to a single vendor and don't care about third-party clients. On the web, every browser is a third-party. For HTTP APIs to take off, they need to be built more like websites, or else vendor specs will fill every niche.