> Unless the system literally prevents you from installing a competing browser, you can always just go and download the competing browser and use it.
Not if the company providing the competing browser goes out of business and stops offering it.
That's really what antitrust regulation is for: to prevent one dominant company from killing all their competitors. Once that happens, the dominant company has no more pressure to succeed and their product stagnates, harming everyone.
Browsers are actually a great example of this; despite the government prosecution, Microsoft succeeded in killing off Netscape and then had the entire browser market to themselves. The result was IE6 and "made for Internet Explorer", a horrible mess that took years to clean up.
