Unless we’re all going to be developers, I don’t see how JAMstack can oppose WordPress. WordPress is (was?) a DIY technology, that empowers everyone to build on the web and publish their content (on their own, on their server). You don’t have to know a single programming language (it helps, but isn’t mandatory). JAMstack is a highly specialised technology, ran by developers. Sure, you can do it without being a developer tout court, but that’ll be the exception. There’s no way you can build a website in a day with JAMstack without knowing some programming language and the way all this stuff glues together (services, APIs, …).

WordPress and JAMstack can work together, but there’s still a long way to make it user friendly. IMHO, WordPress needs to care both to the entry-level user, improving its UI/UX and overall experience, and the developer that embraces new and future technologies (but still needs to rely on a proven user-friendly way to get things done, content published, etc.).

Not that it matters, but I still believe the transition to WP 5 and the introduction of Gutenberg should have been done with some kind of fork, which could allow WordPress to overcome some of its tech debt. It would be a risk, of course. But, maybe, it would allow to bring in more developers interested in newer technologies, and to integrate them more organically and with less need to reconcile with old code and methods. It would have opened up the field, instead of being almost just within the community itself. Also, it could have captivated people who are interested in other ways of publishing content, which is, in my opinion, the biggest challenge that lies ahead.