They become a monopoly when they own everything or otherwise dictate every factit of computing. I don't see how it is necessary to to own all of the pies.
If they could dictate every facet of computing they would definitely be a monopoly. However, Google is more dominant in the Internet search business than Microsoft, and it's hard to believe that Microsoft buying the struggling Yahoo would suddenly make them not only dominant, but able to control the market.
There are advantages to consumers to having a company extend it's expertise in multiple related markets. It cuts down the costs of development, and can help produce innovation, AS LONG AS THOSE MARKETS REMAIN COMPETITIVE.
Even if one company does become dominant in a market through innovation and competition, that isn't illegal unless they take illegal steps to prevent others from competing.
Strangely enough things that are considered normal competitive practices in a competitive market become illegal, anti-competitive practices for a company that is deemed a monopoly. Once a company is determined to be a monopoly, it's competitors can still engage in such business practices, while the monopoly cannot.
In the Microsoft antitrust trial company after company accused Microsoft of business practices that resembled their own business practices. Their argument was that they weren't a monopoly, so they were allowed, but Microsoft was a monopoly, so the government needed to step in and prevent Microsoft from doing those same things. They wanted the government to place additional restrictions on Microsoft so they had an advantage over Microsoft.
That left me with a rather low opinion of our anti-trust laws, and an especially low opinion of their enforcement by the Clinton administration, which was basically pushing socialism light through anti-trust law.