"I quit watching when they dropped showings of the Canadian sitcom Corner Gas."
That actually wasn't WGN America's fault. Corner Gas just went out of production after a half dozen seasons on Canada's CTV network. They did 107 episodes between 2004 and 2009, and that was that. Could have gone on, but Brent Butt, the star/creator/producer, thought it had creatively run out of gas (pun intended). He went on to do another comedy series for CTV, Hiccups, which hasn't been seen in the US yet (and which is on hiatus after two seasons, maybe to return, maybe not).
Lots of networks take a pounding when a key show shuts down of its own accord or a key star walks away. CBS struggled after its best sitcoms of the 70s (All In The Family, Mary Tyler Moore, MASH and Newhart) all faded from the scene between 1977 and 1982. ABC took a hit after Happy Days and Laverne and Shirley, which helped it to the top in the mid-70s, faded about a decade later. Each of those networks eventually recovered. The biggest example, and the one example of a depature that wounded a network long-term so much it is still struggling over a decade later, is what has happened to NBC since the end of the 90s after Jerry Seinfeld pulled the plug on his super-hit sitcom. NBC's parent company GE tried so hard to convince Jerry to do just one final tenth (1998-99) season, they offered him and co-creator Larry David each $110 million in cash, plus stock and a seat for each man on the GE board of directors, not to mention boatloads of cash to his three on-air co-stars. They decided no--saying they had maybe a half season's worth of good episode ideas left at most, and (maybe thinking of longterm syndication values, which have made each of them billionaires) believed it's better to leave a half-season too soon than a season too late after the creative tank's empty and showing it. NBC for its part was hoping Seinfeld would buy the network enough time to develop another killer sitcom...and maybe attract and encourage more creative talent to give the network a shot.
NBC has never been the same since, and that's one reason GE bailed out and sold the network off to Comcast.