Simple patches/upgrades vs tricking people into thinking you've made a whole new piece of software. Linux, BSD, and Apple roll out OS upgrades with new functionality without charging for the new versions.
That's one perspective I suppose. I have a MacBook on my desk at work solely for testing in Safari. I can no longer use it for that purpose because it won't even let me upgrade the OS. That sounds like a whole new piece of software to me. Windows actually has been substantially re-written. I guess MacOS has also? It seems more honest to me call it a different product.