We have fifteen years worth of client and proxy implementations that do not support GET with a request body. This ill-conceived spec change violates "be conservative in what you send" just to avoid adding a new idempotent request method that actually defines usable semantics for its body.
If I had to consume an API with entity-body-having GETs, I would definitely conclude the designers had no practical experience in the field, and had read about this obscure possibility somewhere...
