Actually you are quite wrong. HTTP already accommodaties nonstandard methods, so HTTP-compliant servers do support whatever string you put together as the method.
What you are failing to understand is that this proposal defines both a method and its semantics. This means the expected behavior regarding idempotency, safety, cacheabiliry. Nonstandard methods by design are interpreted as being unsafe.
What you are also failing to understand is that GET is explicitly designed to not have a request body. This means that a GET with a body is interpreted as something that violates specifications and is potentially an attack such as request smuggling. Again, some API gateways strip them as a security precaution.
> Most of the systems that are blocking GET/body could be easily tweaked to allow it. Today. As is.
Utter nonsense. You are talking about things like home routers and old phones.
> QUERY will likely need firmware updates, core engine updates, etc.
Not really, only if those devices do not comply with HTTP.
The real value proposition is the semantics of a QUERY operarion regarding safety and caching. It's not a coincidence that this proposal is backed by the likes of Cloudflare. All HTTP compliant requests are just forwarded through all internet boxes, and the likes of Cloudflare sits at the edge safely caching them.