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It is indeed the compatibility that is the worse issue. The problem is what people have gotten used to and may have coded their applications to expect/deal with. I agree with you that most people would *not* find it surprising to see the name reflected in the OS, but I don't think the convenience of that is worth introducing a potential backward incompatibility.
On the other hand, I think this might be an appropriate place to use a global control, so that getting thread names out to the OS would require adding just a single line of code to any given application. I know of an application that does this. It chose to implement it as a global change, and that makes sense to me.