That was eventually overturned in court, but it was a decade-long fight. It took another decade before third parties were allowed to sell devices that transmitted sound through the phone system with acoustic coupling, and another decade after that before you could just plug in your own phone.
For everything Google does, there are viable competitors. They may not be nearly as large, but they're extremely easy to switch to if you want. People don't stick with Google because they have no choice (as would be the case with a monopoly) but because they like it more.