>The ability for an application (eg, Office) to install side-by-side with other versions of itself is entirely different than its Backwards Compatibility (eg: ability for current versions to open and/or save Office 2007 format). In fact, because of the latter, provided they did a good job at it, the former is arguably irrelevant.

I am still somewhat surprised that Windows doesn't have a Docker-esque sandboxing system to solve this problem (I mean, it has Docker, but it's based on a full VM). I suppose in the third-party realm at the very least you have Sandboxie, but meh...