I see what you're saying, but hasn't nearly every Apple customer effectively done that? A datacentre could maybe rip out Xserves and replace them with equivalent 1U boxes running some generic Unix apps (e.g. Tomcat) but anyone who has developed on the Mac platform is beholden to Apple.
Not that this is necessarily a bad thing, and not that there is an economic way to stay completely platform agnostic, but it does contradict what you say.
Obviously, it's not guaranteed that you'll be that smart. Detroit should have been smart enough to take their expertise in developing for the petroleum market and transfer it to success in the alternative energy market. But it's still not as bad as having a key component of your product be supplied by a monopoly (just ask your local jeweler).
...so really, it's not so much about hardware vs software vs hardware+software. It's that the smart businessperson doesn't knowingly make a deal with a monopoly.