And if only MS had a similar "never break userspace" rule that applied to even the most unbelievably "casual" of software too.
Hell, I broke four apps just going to 64-bit Windows 8 from... 32-bit Windows 8.
If that happens (and Microsoft is one of the best at not breaking userspace), WIndows development would stop overnight.
Most developers are crap - and I'm sure "never break userspcae" is routinely violated by Linux as well, just it breaks little apps that no one knows about and someone either fixes it or codes some other workaround.
Yes, developers are crap who are more apt to take a shortcut "because it works" over doing it the proper way. On Windows, it's easy - if you run a non-English version of Windows, or put it anywhere other than C, you'll find yourself with a "Program Files" folder soon enough because it was hard coded in over using the system APIs to retrieve it. Or you might end up with a C:\Windows even though Windows is installed on D: purely because someone hardcoded a path there.
Plus, there's tons of legacy code out there - a surprisingly large amount of code is still 16-bit (which breaks on 64-bit), usually more bespoke applications used in specialized areas, but hey, if you ever wondered why there's a 32-bit version of Windows despite most processors sold being 64-bit capable...
And to be honest, a LOT of Windows bloat is due to the compatibility - Microsoft codes around applications that took shortcuts. Apple took the opposite tactic - they refuse to support anything but published APIs - if your program broke because you did something "the easy way" then Apple pretty much says "screw you - you took the shortcut, you profited, now you pay". (And yes, new features often broke poorly-written applications. On Windows, this would mean Microsoft wouldn't introduce the feature, or have to work around it).
And yes, moving to 64-bit Windows breaks stuff - remember what I said about hardcoded apps? "Program Files" for 32-bit turns into "Program Files (x86)", breaking all sorts of stuff.
Vista broke practically everything, which was why it was demonized, but mostly because it showed how poorly Windows apps were developed - all those shortcuts meant ground breaking changes like administrator not being enabled all the time broke a lot of apps that required admin just to run.