What about Apple changing the direction of Flash websites and Flash video on mobile? (when iOS was about 1/10 as big as it is today).
It's been what, 8 years since the iPhone was released, and Flash is still around. I didn't notice a huge decrease in Flash usage when Jobs made the announcement. It doesn't hurt that they have strong competition and Android can still to this day play Flash content if the user wants.
The WebGL exports you currently get from Unity and UE4 are typically huge, slow, and often completely broken. I have no idea where the blame lies, but it's not pretty.