People screw up. The people working on Chrome for iOS didn't mean to break WebP support, and when they discovered this they fixed it in the next revision. They're already plenty motivated and on top of things.

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.)