Heh, is not that easy. The reason is simple: high resolution. iirc, for example, Netflix uses h.265 for 2k+ resolutions. They don't even allow 2k+ on hardware that do not have h.265 hardware decoder for that reason.
Not using high resolution for legacy cases, would be an incentive to update devices to those which support AV1. So it only helps this. Youtube does exactly that already.
> They don't even allow 2k+ on hardware that do not have h.265 hardware decoder for that reason.
So Netflix will swap this requirement from H.265 to AV1. Problem solved.
They don't do it for a long time, because that means they will have to KILL support for TVs, Xbox and laptops that already have 4k support. They just use hardware deciding (because of DRM).
Not kill, will just reduce the resolution for them and limit them to H.264 which they support. That would be an incentive to get a device which supports AV1 sooner if someone really needs that high resolution. I'm sure all AV1 backers will do it at some point.
Most of such devices also support H.264, which can be used as fallback for those who don't yet support AV1. H.265 isn't needed at all.
There will be some period when H.265 will linger around, but it will eventually die out.