[stymaar]

You're missing the point. Using GET with a body is currently unspecified, so of course you're not supposed to do it (though you're not forbidden to either).

But specifying this behavior would get you in the same situation as adding a new method: everything not up to date with the spec will keep behaving poorly but newer system would work.

The only benefit of adding a new method is for marketing/awareness: it may end up getting support faster than the alternative because it sounds as a sexy new thing to implement. This kind of benefit should not be overlooked, but we should also acknowledge its limits: most of enterprise stuff (WAF, frameworks, etc.) are not going to work overnight just because it's a new method instead of a spec change.