I've already implemented webmentions on my site, so I have skin in this game. At a minimum I'll be trying to design and code ways of mitigating abuse before it happens on my own web properties, now that I've opened that door.
But if in the coming months it turns out that abuse prevention remains an optional part of the spec, and people have to bake their own countermeasures (or not) on a site-by-site basis, then the writing will be on the wall.