OS X had its nail in the coffin ages ago. It's called macOS now.

But, really, iOS is just a customized build of OS X. A fork in open source parlance. What Apple's been doing the past few years is basically forcing OS X to do things the way iOS does them so they have less code to maintain. In many ways I'm pretty sure the entire reason Apple's going gung-ho about only supporting 64-bit only code in macOS is because iOS dropped support for 32-bit CPUs a few versions ago. Making both platforms 64-bit only would make Apple's life easier, and that's all that matters.
If Mac dies it won't be because iOS was substantially better. It's because Apple doesn't want to make personal computers anymore. The margins just aren't high enough, and Mac customers think they should be able to hold onto hardware longer than a couple years.