[ElectricPim]

I guess most of that is offered in h264 too, but doesn't that imply that the actual legal supply of vp9 content is bigger then of hevc content?

It implies that VP9 is not widely used. We spent large amounts of engineering time on optimising HEVC.
While that could be done for VP9 it would benefit almost no one. Whereas spending the time on, say 10-bit HEVC would benefit many more users.

It is used by YouTube, apparently with DASH streaming. I doubt YouTube will ever use hevc. It is supported in Chrome, Edge, Firefox, and Opera and Android 4.4+ (and JW Player)

There is also a big technical issue with VP9. With standard encode settings the arithmetic decode of a frame is dependent on the arithmetic decode of the previous frame.
That means that multithreaded decode is very limited compared with HEVC, where we can be decoding a number of different frames concurrently.
A multi-core software decoder will always be more capable with HEVC than with VP9 (when both codecs use the default settings of encoder).

Recommended encoding settings for VP9 nowadays support multi-threading decoding.
http://wiki.webmproject.org/ffmpeg/vp9-encoding-guide

My experience is that decoding VP9 is much faster on x86 than hevc. It's a pity it isn't on ARM architecture. Encoding VP9 though is much slower. I compiled ffmpeg with vp9 support on a rpi2, it is super slow (0.1fps), but much better quality per bitrate then hardware h264 encoding (10fps).

1080p hevc seems the limit for an RPI3, but when VP9 decoding is really 55% faster it feels like a nice option.
Hopefully we will see AV1, and hardware support some day.

I certainly appreciate all your and others efforts on optimising Kodi / HEVC RPI, and I'm not advocating it should shift to VP9, I just wondered, I have the feeling VP9 is underestimated.