I have comcast service in one of the most dense urban environments in the US.

I have no alternative. I am being bled dry because there is no competition of similar service.

Please, if you're going to compare things at least stick to fruit. This is apples and basketballs.

50% of folks in Comcast’s footprint don’t subscribe. Those folks aren’t going without Internet access. For a significant portion of the market, those other alternatives are viable. That creates competition, though obviously less competition than an equivalent competitor. (I’ve been lucky to mostly live in FiOS territory, and now I’ve got two fiber lines to my house. But back when I lived in Wilmington, I had just Comcast. Instead of subscribing, we used a T-Mobile hotspot the whole time. It was fine. With 5G, that phenomenon is only going to increase.)

Pew reported in Jan 2018 that only 65% of adults are home broadband users. That's down from a peak of 73% in 2016. Segmenting by age, fully 50% of people 65 or older do not use broadband internet at home.

So what are they using? Mobile phones--Pew says 20% of adults do not have broadband at home, but own a smartphone. Or nothing--Pew reported that 11% of U.S. adults do not use the Internet at all.

https://www.pewinternet.org/fact-sheet/internet-broadband/

And what % of people that actually have alternatives like FiOS (unlike me) don't subscribe to comcast? Saying 50% of subscribers in Comcast's footprint is prevaricating about the bush AF. Why, you ask? If 50% of potential Comcast subscribers have an alternative to Comcast, that would mean 100% of people with an alternative are taking advantage of the free market. Which would mean that 100% of the people remaining are likely forced to subscribe through a lack of access to alternatives.

I don't even have working voice cell coverage at my house, much less data. Again, almost certainly due to a an entrenched monopoly limiting the market outreach of competition, and thereby obviating the requirement that they themselves perform adequately enough to retain customers.

Q.E.D.

For your hypothetical to hold, you’d expect to see some providers with near 100% market share. (You’d also have to assume Comcast builds and maintains infrastructure to millions of households in areas where it has no market share.) Who are they? FiOS’s market share is 40% in its footprint. AT&T’s fiber is around 25% now, hoping to reach 50% by 2023.

The only other way to get your math to work out is to assume that only a very small portion of Comcast’s footprint has no other cable/fiber competitor.

As to your cell situation—what mechanism do you think creates this competition-limiting monopoly? And why doesn’t that same mechanism apply to say my house (in the DC exurbs), where all of the big 4 have a decent signal? (Or the majority of the country, where at least three providers have coverage?)

So you’re saying the upper bound on the number of municipalities where Comcast has no competition is half of them?

Not saying much at all there.

What percentage of people in Comcast's footprint have broadband internet? 50%/55% is a rather different market share than 50%/95%.

This is such a disingenuous argument.

DSL, cellular, satellite, cable broadband, and fiber broadband are not equivalent.

By your reasoning, as long as POTS exists, dial-up ISPs are a competitive threat.

If changing the product is evidence of competition, then Google Search and Android must have some, since they are continuously being worked on and new versions released.

Responding to a specific price point is the sign of competition, not general improvements in the product. AT&T built a gold-plated, state-of-the-art telephone network in the “Ma Bell” days, without facing competition. (And gave us the transistor and C and UNIX while they were at it. Google in many ways fits the same “benevolent monopoly” mold. I think it would be unwise to take antitrust action against Google for that reason.)

You never used the old long-distance telephone system, did you? The only thing that was gold plated were the executive bathrooms at AT&T.

Verizon may have price matched Google, but Verizon fiber service is only available to a small fraction of the country. The cable companies, which have a much larger footprint than Verizon's fiber, have mostly limited price cuts to markets where they face competition from fiber. Comcast for example still charges $150 for gigabit (1000/35 Mbps) in most markets. By contrast markets were a competitor has built out fiber get the price cut in half. If this isn't a tacit admission that they are abusing their lack of competition by price gouging, I don't know what is.