> heavily lower the ranking (or completely remove from their index) any sites that don't meet basic performance and readability thresholds.

This seems like a much tougher problem than AMP.

From an API design standpoint, it is much easier to build and maintain an endpoint that receives data that must be in a specific, proprietary data format instead of an endpoint that allows the user to input data in any format and then attempts to parse that data after receiving it.