The situation in Parkersburg is kind of contrived (as it is in Harrisonburg, Presque Isle, Alpena, Lima, and I'm assuming, Mankato) where they are able to create the appearance of having several stations, but in reality, there is only one, with only one newsroom, and "fake" network affiliates created by using subchannels and O&O LPTVs. Put another way, there's no competition, and you only have one newscast, and one editorial point of view. It's really a return to the day of multi-network or all-network affiliates made easier by being able to run the full schedule of each network on subchannels, which didn't exist in analog days.
I see what you are saying, but I am just thinking that, for instance, NBC might want to expand the Columbus market, and do a huge favor to WCMH, by pulling the NBC affiliation from WHIZ, in effect telling them "if you can get ABC, CBS, or Fox to affiliate with you --- in spite of the fact that they probably have no desire to dilute the audience of their Columbus affiliates either --- go right ahead, and if you end up only being able to affiliate with something like Comet or TBD, then as far as keeping a separate market, more power to you". People in Muskingum County aren't going to quit watching NBC via WCMH. True, that would be throwing a loyal affiliate under the bus, but that's precisely what they did with WHAG, WMGM, and KENV.
Yes, the markets are probably pretty much etched in stone at this point, and it's only rarely that a county flips anymore. Forced carriage of only one market's stations pretty much erects an electronic Iron Curtain where cable and dish viewers are concerned. Just look at the Myrtle Beach-Florence SC market. The Wilmington and Charleston stations just disappeared, even though they are easily received with rooftop antennas due to the coastal terrain. At one time, Horry County was actually part of the Wilmington DMA.