I'm just going to point back to what @sandhawke said:
> Abandoning your early adopters before establishing a new user base means you have nobody.
The time to suggest a 100% breaking change to the spec has long passed. There is already a lot of written and deployed code that sends webmentions using form-encoded syntax. It's not a matter of simply making a "tiny" change to switch to JSON, since it will affect senders as well as receivers, some of which are using libraries on both sides, not to mention the fact that this software is deployed quite widely and in places where a single person can't even mandate that it be updated. Such is the situation when a spec gets widely implemented.