The future is Next.js with SSR first and selective client side hydration. Ignore the jamstack, it's going to lose and go away. Requiring a JS download first to show a dynamic site is a recipe for bad performance.
1. Manually marking a component for hydration likely means shipping the whole structure below as JS, which may hit diminishing returns fairly quickly.
2. At least if you’re using TypeScript, enough is known (or could/should be) to determine which components are truly static. Next.js already has a rudimentary version of this. That kind of analysis could be a huge DX improvement.
Next.js gives you a great toolbag to choose from for each challenge, it's definitely going to stay around and keep growing like crazy.
JAMStack is Next.js with SSG+CSR.
JAM has other problems. I don't think that's one.