OSX has moved to a free OS upgrade model as of two releases ago and very cheap upgrades for the two releases before that (the last 5 years of OSX upgrades cost less than any single Windows OEM upgrade of the same time period). The most recent release supports most of their computers from 2009 and much of the more-capable hardware from 2007 (basically all the way back to the first Intel-based Pro Mac lines).

Yeah, it's shorter than MS and Linux support windows but I wouldn't describe it as "constantly buying upgrades" either.