Considering how many viewers they have using televisions/set top boxes/streaming usb sticks that will never get an update, do you see that happening any time soon?
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I believe they will be very reticent to cut off access to this long tail of users with outdated computers, with no account on shared IPs.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.
I would expect them to simply limit streaming quality without having an account to their standards. They tried locking 4K video behind a paywall before and people were upset, but locking it behind an account (plus WEI, but 99% of users won't know the difference) seems like it wouldn't cause outrage.
You'd be surprised, I had some old Symbian phone with some really ancient Youtube app and it still was working couple years ago. The phone's screen eventually died, so I can't check now
