Msft couldn't have bought Apple. They needed to have plausible competition in the desktop market to avoid monopoly regulation, etc.

There's a good chance Apple wouldn't exist without Microsoft (and vice versa):

http://en.wikipedia.org/wiki/History_of_Apple_Inc.#Microsoft...

Without Office, etc on Mac the adoption could have been severely limited. Would Apple have had the cash to develop the iPod?

I think of them as competitive symbiotes. Together they are more valuable than the sum of the parts.