Well, certainly in developing countries where bandwidth is limited, you're going to be watching a small resolution, and any amount of CPU is fine so long as you can cut the bitrate.

There are a lot of people in places like that.

Also, Allwinner, Intel, Mediatek, and Rockchip all have SoCs announced with hardware VP9 decode: http://wiki.webmproject.org/hardware/socs (not sure how many of those are already shipping).

There's a talk online by a Youtube engineer that covers them rolling out VP9.

Apparently the stats on user engagement show improvement. It seems that initially they showed a lot of improvement, but then they realised they were comparing the average video against only the most popular videos in VP9 (since those made the most sense to transcode first) and obviously people are more likely to keep watching something really popular.

Once they took that into account they did identify that some older machines would have a degraded performance and therefore lower engagement with VP9, but the heuristic for avoiding them is "Running XP" so it seems that most modern machines have a better experience with CPU decoding of VP9.

I believe they were up to 60% of all desktop views in VP9 by the end of last summer, so I'd guess if it was going to melt anyones's computer it would have happened by now.