Youtube wants to be a ubiquitous presence in modern culture, therefore it is in their interest to continue to support all the old and busted computers/etc that are still in common use. This includes allowing people to watch videos without an account using the old busted up computer they have in the back room at work, keeping youtube accessible on publicly shared IPs like libraries, coffee shops, even VPNs.

I believe they will be very reticent to cut off access to this long tail of users with outdated computers, with no account on shared IPs. As long as this is true, it will be possible to use youtube in contradiction to their TOS. They could lock things down quite tight overnight by requiring people to sign in and use up to date browsers/players, but doing so would threaten their ubiquity.