You need to read more history – that naive dismissal doesn't work well for anything important. If someone corners the market on, say, oil, cargo transportation, etc. your choice is either to pay the monopoly tax or accept a significant loss. Do you really think “oh, it's just slightly less effective” is why the Sherman Antitrust Act (
https://en.wikipedia.org/wiki/Sherman_Antitrust_Act) was seen as widely needed by basically everyone other than Standard Oil? Similar stories apply to anyone who tried to make a living when the railroads had significant regional control – it wasn't a slight hit, it was “do you want to sell your goods outside of your town or not?”
(All of these are well known, well documented examples a little over a century old. For a more modern alternative, consider the many people who have a single provider for internet access – they can either pay or give up on significant participation in the modern economy.)
> Also, a private-sector monopoly, no matter how seemingly entrenched, is always vulnerable to disruption (when was the last time you used a PC manufactured by IBM? Or were forced to use IE against your will, for that matter?
Amusingly, both of your examples are cases where the monopoly was disrupted under the treat of government action. The reason those other browsers had a chance is that Microsoft couldn't lock down the PC market as much as they wanted to while the DOJ was investigating. Similarly, look at IBM's history – do you think the PC market would be as open if they hadn't had to worry about a replay of the 1969 antitrust settlement?