Also, it’s not the whole picture. AV1 is open because it didn’t have the good stuff (newly patented things) and as such I also wouldn’t say it’s the most modern.
AV1 definitely is missing some techniques patented by h264 and h265, but AV2 is coming around now that all the h264 innovations are patent free (and now that there's been another decade of research into new cutting edge techniques for it).
AV1 is good enough that the cost of not licensing might outweigh the cost of higher bandwidth. And it sounds like Netflix agrees with that.
h264 is a very good codec.
in double blind testing, at the same bitrate, you'll pick the "new" codec every time. but yes, they're trying to save money on bandwidth. it's annoying