They often do have a functional monopoly on 'broadband' as defined by the government. In my case I'm in a Time Warner area that may or may not become part of Comcast when they merge (and I have no doubt they will get the ok). My other options are 1mb/512k ADSL (which is internet access, but not 'broadband') from Verizon or 3G internet access from a number of cell providers (4G does not currently exist in this market of over 100,000 people) which all have considerably low caps that make them nonfunctional as primary internet sources. I probably could find satellite internet as well, but for extreme prices and limited capability that is even less 'broadband' than my other options..
So yes, I could have 'internet' from other people (most of whom are equally offensive)... But that is not a choice. I'm positive if Level 3 did shut the links to Comcast off that Verizon, AT&T, and at least three other providers would be turned off at the same time and that would cover 90% of my options.
I need internet for streaming media, general internet access, email, cloud storage, and gaming. Only one company allows me to do that effectively and even if I did switch to a worse service I'd lose the ability to do some of those.