Also, this looks like a bleak picture for the JAMStack if Wordpress keeps growing no?
The JAMStack has a usability problem. That's why wordpress keeps growing. What do i need for wordpress? some cheap(maybe even free) webspace and a database. updates to the wordpress core are quite simple doable via webinterface. done. with this even the biggest simpleton can write blogposts.
with a jam stack? headless cmses, blogposts written in markdown, config in "frontmatter", building and deploying your blog, versioning via git, not even a litte autocomplete. even i recommend wordpress because i have not the time and nerves to teach non-it people to use the JAMStack.
I got rid of it on my own website some time ago but the startup I'm with still uses it. It's great; doesn't need my attention and our marketing people can deal with content. I'll likely transition it over to a managed solution at some point but beyond that I don't have the bandwidth to building and maintaining a new company website.
That said, newer CMS's have their work cut out for them.
Sure, but no-one but very technical minded people would pick those instead of just WP. It's just extra complexity which you don't need when you start a blog or a shop or something like that. I would have a hard time defending it unless in very specific situations; wp is just too stable, too many plugins, too many programmers can support it, designers can make templates for it with ease etc, all hosters support it for next to nothing etc and without setup.