Yeah, sorry, i didn't mean to say that the browser engineers don't have other motivations to not break shit, or to fix it when it's broken, but knowing that all the websites aren't going to roll over and accept whatever the browser vendors break is a good motivation.
It doesn't make sense to punish visitors for the mistakes of browser makers, even if browser makers need more pressure, which they don't.
(And the situation with Opera is even trickier. They did support basic WebP and no one was even doing Accept-based detection yet. More communication would have been helpful between Chrome and Opera to coordinate what "Accept: webp" would mean, but it's not a situation punishment would improve.)