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But those websites suck down enormous amounts of data each time they load.

Not if they use caching (whether service worker based or not) aggressively.

If an organization isn't taking the time and effort to make sure their app isn't bloated full of stuff, what makes you think they'll do that?

caching can be done reasonably well for straightforward cases with yet-another-library https://serviceworke.rs/

oh, the irony!