YouTube's compatibility hacks are against buggy versions of browsers. Regardless of whether a bug is eventually fixed, once it is released, there is some segment of the user base that has it. If that segment is large enough, and the bug easy enough to hack around, then YouTube will provide a workaround rather than lose revenue.
Sounds like the ultimate solution here is to just give browsers a killswitch, so they'll eventually just stop working if you don't keep them up-to-date. (I'm not sure whether I'm joking.)
