Personal opinion hat on... again, personal opinion only here... these do not in any way, shape, or form, represent opinions of the company that I work for.
As a software developer, back compat just plain sucks. It stresses test organizations so hard it's unbelieveable - test case matrices can triple in size, configs that need to be tested get huge (in the case of non-O/S software, we're talking things like x86, x64, WoW, different O/Ses, etc), side-by-side configurations are costly, etc. It is impossible to hit everything. Developers hate it because it creates spaghetti code and forces them to maintain ancient codepaths that see little code coverage. Program managers hate it because it limits the development of new features. And then after all that work, someone will complain that their scenarios don't work and requests a QFE. Without question, it is the most un-fun part of development.
And really, in the end it's the customer that suffers because perf is what usually suffers. Back compat comes with a price.