Office and Azure make up half of their revenue, but their other revenue streams are actually different things, like Windows, gaming, LinkedIn, Bing advertising. It's actual diversification.
With the introduction of IE in 1995 and its deep integration into Win95 and ruthless take-no-prisoners assault on Netscape, Microsoft was probably already aware that they were starting to resemble a monopoly, given Apple's poor sales and market share at the time. My personal take is that MS propped up Apple to help appear to have a competitor ahead of the Antitrust action in 2001[2].
[1]https://www.cnbc.com/2017/08/29/steve-jobs-and-bill-gates-wh...
[2]https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor....
I'm _pretty_ sure Apple could have gotten the money from elsewhere if they really had to. It's history now, so we'll never know for sure, but I'd bet that they could have found other investors if they really had to. People seem to think it's a black and white scenario - money from MS or death! But I don't think it ever was.