"No one ever got fired for buying IBM" probably pre-dates the creation of Microsoft.

The saying was based on the fact that IBM, like the 7th Cavalry, would ride in to rescue large accounts if they screwed up.

I never heard it said of Microsoft in the 90s because Microsoft was never a big proportion of anybody's spending. It sold to hundreds of millions of consumers and small companies, rather than Fortune 500 companies.

I did once discuss this with one of the Microsoft salesmen who handled giant corporations, and who had been hired from IBM. He said IBM tried to get 33% of a company's IT sending whereas Microsoft tried to get 3%.