You can, its actually easier than in Windows since theres no licensing issues to handle.

Windows doesnt really have licensing issues. You call a phone number, tell them you want to activate windows, read them a number, and a machine reads you a number. It takes about 6 minutes of your life, and Microsoft always activates when you ask.

For example, if you buy Windows 8.1 you have downgrade rights to every earlier version. All you do is speak with a person, say "I would like to activate windows" and they say ok. Its not a hassle unless you do it daily.

It's a hassle if you have to call the vendor at all. Not all Windows versions have downgrade rights (see Vista, certain editions of 7).

Also pricing is an issue: all recent desktop versions of MacOSX are either free or very inexpensive ($30 or less).

You could likewise say that filing an insurance claim is easy... sure, but for some folks even knowing to make the call is an undue burden - exactly how the vendor wants it.

As a counter argument, Apple fans are always telling me "just go to the genius bar, they'll sort it out". As if that isn't a hassle. I've never had the need to take any of my windows machines in to be serviced.

Old versions of OS X that you've previously downloaded are always available for re-download in the Mac App Store. You can use those downloads to downgrade.