[Someone]

I think this is a slightly different 'backwards compatibility' than uepsed elsewhere. I bet Mathematica has seen zillions of incompatible changes. For example, http://www.wolfram.com/mathematica/quick-revision-history.ht... lists "Many new automatic simplifications for derived distributions, including affine transformations, sums of variables, parameter mixtures, and censored and truncated distributions" for 8.0.1. I haven't used it, but I bet that there still is only one "Simplify" function, and that there aren't a zillion settings to make it behave 'just like version X for any X in 1.0, 1.01, …, 8.0.

Compare that to e.g. windows, with its BlaEx, BlaEx2, etc functions, or a typical database, where a zillion flags control what language dialect they follow, how exactly NULLs behave, etc, or filesystems (for example, Apple's HFS has a bug in its filename sorting that has been faithfully copied for decades now)