What's missing is a way to pin web apps so that you always get the previous version (and can opt in to subsequent versions after checking their hash from a trusted source).

There is a clever way of doing this, using a bookmarklet, a dataURI, and SRI, but the UX isn't great.[0] If something like Hashlinks[1] were supported by browsers, though, this could work quite nicely.

[0] https://news.ycombinator.com/item?id=17776456

[1] https://w3c-ccg.github.io/hashlink/