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I don't think this is true. Most web images shouldn't be "near lossless" they should be "not ugly", with the exception being images that are core content (think Flickr or Unsplash).Hard disagree here. We're talking image format, not image renderers. Whatever ends up being standardized on will eventually become the format "core content" is being stored in and transferred in. It will be the format in which my phone will save photos of my daughter, the format that her grandparents will receive those photos in (possibly twice transcoded by whatever IM they're using these days), so believe me, "not ugly" isn't gonna cut it here.
The photographers, the stock photo sites, the marketers - they'll all know how to keep images at high quality, and they have their reasons for doing so. Regular people will typically just use the defaults, so it's important to optimize the defaults for them, instead of marketing spam. If marketing spam is indeed the majority of data traffic on-line, there are other avenues to address it.
> I did a performance analysis of a set of sites recently, and many have images that are 10x bigger than necessary
Yeah, they also don't care. Why would they? Bandwidth is usually too cheap to meter for most commercial sites like these. If we try to fix their image sizes top-down by more aggressive quality reduction, some marketer will eventually notice the images "look bad", and fix it. Meanwhile, regular people will get the short end of the stick.