In the end, I think technology adoption by stealth is the only way to make an idea dominant. With the open web (I use that term intentionally since “Indie Web” is much more of a community I don’t know well), people weren’t excited to adopt http. They were excited about websites. Older open web technologies, like trackbacks and pingbacks, weren’t successful because of the technology, per se, but instead of the ease in which regular end users could use it to their end.
In adoption in WordPress Core or Jetpack, the technology would need to not seem like technology, but only a feature. “Automatically notify sites you’re linking to about your post” is the feature, which exists in WordPress already through now-antiquated tech, so for webmentions to have a life in Core, they would need to augment that feature. “WordPress now features rich notifications”, for example.
I do fear that the dependence on microformats will be a detriment to widescale adoption, at least via the WordPress vector. For better or worse, markup is theme territory. Genesis already adopted Schema.org, mainly due to the SEO benefits given schema.org is Google-backed. _s has been hesitant to pick sides. If either method is going to get traction, building it into a default theme and, possibly, a
theme_support declaration would be the way it happens. If that happened, then other theme authors may opt to build support and the snowball would begin to roll.
I don’t know the webmention spec at all so ignore me if this is ignorant. Instead of webmentions relying so much on microformats and scraping, what if it could use either microformats or a reference to an API. WordPress now has a REST API, so what if the webmention allowed an alternative data location for structured data? When available, if the receiving site could verify the mention and then pull from the API, then it would likely be more reliable and not markup dependent, thus more likely that Jetpack or WordPress core, etc could add support for it.
In short and to the point, for something like this to gain traction, it would need to just work in a way that reduces fragility and developer error.