In regards to X and it's growing pains, in the era of source control, why is backwards compatibility with ancient systems still a concern? If you're trying to put together an old OS for some old hardware, why not take it out of the source history and let the head progress forward unhindered?
I would guess it's because the moment you do that you push down the problem to the library/application programmers. They will still have to support old systems and suddenly it's no longer a handful X developers who have to maintain backward compatibility but thousands of application developers all over the world which have this task. And certainly not all application programmers will care so the problem will be pushed on further and hit some users.
The problem with downward compatibility never completely goes away. But the closer it is handled to the base the less people have to care about it.
Now there is on the other hand a point where you could say every application programmer has to do so much extra work because the protocols are outdated and overly complicated that breaking downward compatibility will make the life of the average application developer easier. Not an exactly defined point in time - definitely way less obvious than the moment where compatibility is broken. But I guess that's when the break should (have) happen(ed).
I get the impression that many Linux users have specific old versions of software (often a window manager, which complicates the issue) that they want to keep using forever, but want to simultaneously use the latest version of some other software (like a browser).
Linux is pertinent here because we're talking about X11, but it should be noted that this is by no means specific to Linux - the same could be said about any OS with widespread use by different groups of people.
