I see the appeal of underscores, and I may come around on it, but there's something I think is worth mentioning, and it applies to this and a number of other proposals...
All changes to the spec that are something other than a superset of what it is now, have the potential of fracturing the available tools. I personally will most likely update WJElement to validate according to post-v3 drafts, but that would be a hard sell if it would affect customer data or integrations, for example. Some current tools will have a supported base going forward that for one reason or another will make major spec revisions very problematic to implement.
Now, maybe that's not a problem worth worrying about. I was a pretty early adopter of JSON Schema, and this stuff comes with the territory for early implementations. And even if there were tons of widely-used tools out there already for v3 of the spec, just look at Python. Old and new versions can coexist if need be.
"Perfect" shouldn't be the enemy of "good enough". The v3 spec was good enough for a decent collection of useful code to have grown up around it. It's quirky, but it helps us get the job done. Like the x86 architecture, or Bash, or JavaScript itself. Growing slowly and organically, even if it's uglier, has its advantages. Those things are not elegant, but they're everywhere. Food for thought. :^)