The challenge as I see it, and the reason I don't use them, is that retail themes do not support microformat 2. Without microformat 2 support and semantic markup, the IndieWeb tools and technologies such as Webmention are pointless. Until the microformat 2 issue is resolved, I don't see any point in doing the work to support blocks.
@manton WordPress has been my blogging platform for over 15 years. I also don't want to see WordPress dominate to the point where other platforms have no chance of success. We have Ghost, Grav, Tumblr, Square Space, Blogger, Micro.blog and way-too-many static file generators. I want to see that kind of diversity continue.