(Note that I am aware that for many pages you can, in fact, do this already. Downloading HTML is a thing. I just meant that since I can already do this, it would be nice if I could both do this and have it integrate seamlessly with my browser for navigating links and host those files for other people while on WiFi I was at it)

This is the dream all us IPFS/decentralization nuthouses are chasing.

Idunno, "Save as HTML..." seems to go for that dream pretty well. And lots of other sites utilize service workers fairly well to work offline.

Save as HTML is often completely broken. Especially when javascript injects <script> tags to load even more javascript. You're better off by using archive.org's wayback machine.

There is a number of other offline tools available (google "archive team warc ecosystem"), they just need better integration.