Just use a different browser for VP8/VP9 stream decoding its not the end of the world.
Apple invested heavily into MP4 software and hardware (I believe with the iPhone in 2007). They pushed the format hard to make it a standard and it worked very well.
It was until 3 years later that Google launched WebM with the VP8 codec they purchased. Took another year for native Android support, and another year to see real hardware support. Plus the hardware needed to be adopted by the consumer in their normal upgrade cycle. So 5-6 years after people were using MS and Appl's standard VP8 was reaching similar efficiency levels?
Even today we are seeing about a generation between h265 hardware support vs VP9 support (Intel, AMD, Nvidia, etc).
Adding native support for WebMs VP9 codec is no trivial task either. Like the HEVC/h265 codec VP9 is capable of some very complex encodes. Its not a simple file that deficiencies in the encoder don't matter because the CPU can power through. There is years of work to get it on par with Apples implementation of h265 and indefinite amount of time for maintenance.
VP8 is supported in WebRTC on Mac which is an API used for things like video conferencing. This should NOT be confused with WebM support for VP8 playback as that is still not supported.
This is my 2013 iMac (Haswell) playing the same VP9 and h265 encoded video. The video is 4k 60hz 10 bit played with IINA which is software decoding since this iMac doesn't have hardware support for either of those codec. I intentionally used a file that would be difficult to decode to see variances in performance.
VP9
Unwatchable, its probably playing at around 5fps. This CPU just isn't powerful enough to decode the complexity of this file in real time.
H265
While still heavy on the CPU the video plays smooth. Keep in mind this is being done through software on a 2013 iMac.
This is the same video on my 2018 13" MBP (Coffee Lake) using hardware acceleration.
VP9
Plays smooth as expected, still heavy on the CPU comparatively.
h265
Again as expected, plays smooth although plenty of resources left.
That doesn't "prove" anything except for how much work Apple put into HEVC/h265. A year ago that video wouldn't have played at all but through software improvements I can make it work. While I have no objections with VP9 support in MacOS and Safari I don't really see Apple doing something that they've already done once for...and lets face it....pretty much exclusively 4k youtube videos.
Admittedly and comparatively speaking, 4k youtube on my MBP looks brilliant.