@mib2berlin: There are not thousands of codecs but thousands of combinations of settings for all kinds of material and that's what causes the problems (we have over 8k tests in the testsuite).
Apart from that: .h264 will stay for a loooong time simply because most of the cameras or video production etc support it, because every re-encoding lowers the quality apart from if you work lossless (which most youtubers etc don't do because they don't have the necessary hardware) and because it will take a long time until hardware decoding support will come to AV1 (first chips in significant numbers are estimated for Spring 2019 - and I doubt all people will change their hardware immediately). I did some tests yesterday and it needs a quite powerful CPU to decode higher resolution AV1 for now, whereas VP8 and h264 can use the GPU, which is much better suited for such purposes. Additionally buffering on AV1 doesn't seem to work well yet, at least it made some problems with higher resolutions here - especially speaking of >1080p@60Hz - the connection was not good enough to keep up a continuous stream.