The point I'm sure the grandparent was making is that adoption is slow, it was already slow in 2000 and it's slower now. If you introduce one new cool thing, and everybody agrees that it's cool, and they work hard to get it widely adopted, you may be able to rely upon it in a decade.
But as you introduce more new things that most people don't have, the resistance to them tends to sum, some people who (for rational reasons or not) object to WebM will automatically carry that over to WebP, in addition to people who may see WebP as unnecessary.
IMO WebP is another HTTP/2.0, an "improvement" in technical terms which can't deliver enough benefits to justify the short term pain. Video was a special case - the lack of middle ground between "Theora sucks, and I don't care about patents" and "Patents suck, and Theora's fine" left a niche for WebM that may (it remains to be seen) make it mainstream. Also Google has Youtube, and there is no Youtube equivalent for still images, the most popular image hosting sites contain a small fraction of the web's images, whereas Youtube dominates web video.