Honestly, as much as I avoid it as a primary browser, I think all OSes should have a default browser that one could use as a preinstalled webview. Some way to avoid having to ship a 100 MB electron with simple web apps.
I don't think web apps are ideal, but having edge installed and available as a native webview is relevant.
I don't think this dream works yet, I don't think one can actually make THESE kinds of apps without shipping electron yet, but that should definitely be an option. I have no problem with edge used as a random weview in random apps. I only hate it when edge is opened as a dedicated browser.
I don't think an OS API to use Firefox as a webview is a reasonable ask :(.