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. what people have gotten used to.  I agree with you that most people would *not* find it surprising to see the name reflect 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.