As long as the naming pattern is distinct from important domains you can still block it based on pattern matching. They need to obfuscate ad domains and other hosting domains the same way.
Creating subdomains is quite fast because the request goes right through when it's unknown to caches, it's updates when you reuse existing ones that causes trouble with lag.
There's already addons that can recognize in-video sponsored content and skip, if youtube splices in ads into the video stream these addons will still work (although depending on how strict server side logic is, they may have to pause when the buffer runs out until the time of the ad length has passed)