This is quite fair point, yet I'd argue that still most so called web apps could (and should) use basic old-school HTML forms as the underlying technology, and progressively enhance from that baseline, up to the "app-y" look and feel we know and ~~ha~~__love__.

Obviously, there are some limits where application built with bog standard HTML forms becomes too cumbersome or makes no practical sense, as you say, but I think that that threshold is far higher than what current web-app landscape exhibits. I think the threshold is around video editing software, or real-time multi-user collaborative spaces perhaps. But for the rest, following three old steps

    Make it work — just HTML.
    Make it nice — add some CSS.
    Make UX slick —  add JS.

still makes sense to me. In context of the plainvanillaweb.com it would mean just moving the content from non-semantic attributes of custom components to their semantic initial content, such as adapt

    <x-avatar src="https://i.pravatar.cc/150?u=a042581f4e29026024d"></x-avatar>

to get the data from its contents:

    <x-avatar><img src="https://i.pravatar.cc/150?u=a042581f4e29026024d" alt></x-avatar>