The fact is Windows is so far ahead in terms of backwards compatibility that at this point that its not even funny.
It's clear that you haven't actually tried this little experiment.
Even if it were true, here's something you apparently haven't caught on to: I can completely legally and trivially install the old Linux in a virtual machine under the new Linux, and run as many copies of it as I want. I can't do that with Windows. Oh sure, I run an XP VM under Windows 7 anyway, but I wouldn't want to count on it in a business context.
On Windows, it is __TRIVIAL__ to make it so that you unzip an exe built in 2015 to a folder and run it on pretty much every single x86-based windows flavor since Windows 95.
On Linux, it is trivial to make it so that you run a shar file and run it on any Linux anyone is actually running today. The upgrades are free (or part of your contract, I suppose, in the case of RHEL) so people upgrade, and you don't have to support Windows 95. Meanwhile, lots of software which runs fine on Windows XP won't run on Windows 7, and some of it won't even run in "XP Mode" because Virtual PC is virtually worthless. Most of it runs fine in vmware, though. Sadly, Microsoft isn't actually capable of making VM hosting software which will properly host Windows on windows, let alone arbitrary operating systems, nor buying one.