What killed "mining in the browser", more than anything else, was:

1) It was almost exclusively used for malicious purposes. Very few legitimate web sites used cryptominers, and it was never considered a viable substitute for display advertising; it was primarily deployed on hacked web sites. Browser vendors were relatively slow to react; many of the first movers were actually antivirus/antimalware vendors adding blocks on cryptominer scripts and domains.

2) The most popular cryptominer scripts, like Coinhive, all mined the Monero coin. (Most other cryptocurrencies were impractical to mine without hardware acceleration.) Monero prices were at an all-time high at the time; when Monero prices crashed in late 2018, the revenue from running cryptominer scripts dropped dramatically, making these scripts much less profitable to run. (This is ultimately what led Coinhive to shut down.)

I guess slow/fast is subjective. It didn't seem like enough time passed for a legitimate ecosystem to develop. Just the basic idea of say hosting a static-site/blog on a VPS with a cryptominer that could pay for itself would have been a game changer - but was probably just the tip of the iceberg of possibilities. Instead we're still stuck either having to sell our traffic/info to Google/Microsoft, put up ads, pay for it out of pocket. The entrenched players won

The hacked site boogieman felt overblown (and from what you're saying it sounds like if would have died out anyway). I'm sure it happened, but at least personally I never once came across it. Or if I did, then my CPU spun a bit more and I didn't notice. No real harm done.

More fundamentally we're now in territory where the browser vendors get to decide what javascript is okay to run and which isn't.

> I guess slow/fast is subjective. It didn't seem like enough time passed for a legitimate ecosystem to develop.

Coinhive was live for about two years (2017 - 2019), and it basically ran the whole course from exciting new tech to widely abused to dead over that time.

> The hacked site boogieman felt overblown...

Troy Hunt acquired several of the Coinhive domains in 2021 -- two years after the service shut down -- and it was still getting hundreds of thousands of requests a day, mostly from compromised web sites and/or infected routers. It was a serious problem, albeit one which mostly affected smaller and poorly maintained web sites.

https://www.troyhunt.com/i-now-own-the-coinhive-domain-heres...