Another possibility: online libraries, such as archive.org, could store FLAC only to save space and still provide in-browser streaming.

That makes some sense, yes. But so far I didn't see any stores which sell only FLACs.

That's a chicken and egg problem; if the store only provides FLAC, the browser can't play it, so the potential customers can't try it, so they don't buy it, so the store has to provide non-FLAC alternatives, so there is no need to provide FLAC decoding in the browser, etc... Somebody has to make the first step, and I'm happy Firefox did this.

Bandcamp is one of the few stores that does sell FLAC, maybe they'll be the one going that direction.

Hardwax[1] is selling aiff, which can be packed into flac. No metadata though, which is kind of a bummer. Beatport[2] offers wav and aiff as well.

[1] https://hardwax.com/ [2] https://www.beatport.com/