"A cap is usually better understood as a threshold after which the user is subject to a different set of conditions for access, such as movement to a higher priced tier, different product or different speeds."
If after reacing the 200 GB or so the company would provide a slower connection (say 1 Mbit/s), that would be a flexible data service. But cutting access to 0 Mbit/s is a data service cap.
"A current TWC subscriber would be moving from a TWC monopoly in service to a Comcast monopoly in service."
Well, yes. But residential subscribers aren't the only customers. Internet companies are also customers.
Suppose that Facebook has a project to install a super connection between their servers and Internet providers. If there's seven internet providers with each under 20% of the market, the project is tricky to do.
But if Time Warner Comcast had 40% of the market, Facebook can choose to connect to them only and ignore the rest of the internet providers. This is a huge advantage for TWC, and kills their rivals. In addition, TWC could set a very high price for the deal, whereas in a less concentrated market each provider has much less bargaining power.
"nobody should be allowed to own more than, say, 5% market share of the media spac"
That wouldn't do much to competition: companies would distribute markets so each gets a few neighbourhoods or towns.
The solution must be to encourage companies to enter markets where where are competitors. For example, Comcast-Time Warner should transfer half their customers from a few key markets to companies that don't operate there.