Match Game 79 was indeed an embarrassing last season of the CBS run.
To understand why, you have to go back a couple of seasons: Richard Dawson had begun hosting Family Feud in 1976 (legend has it that FF was designed specifically for him, based on how well he had performed in the bonus "Super Match" on MG) and managed to appear on both shows for a couple of years. CBS, however, started tinkering with MG's time slot ... first moving it up a half-hour, then disastrously moving it to mornings in the fall of 1977. That move nearly killed it right there as the ratings plummeted with the absence of the core viewers, students who watched it when they got home from school in the afternoons, and Family Feud took the title of most-watched game show as a result. The network tried to fix the mistake within weeks but MG ended up at 4:00pm, a slot which was increasingly being pre-empted for local programming by the affiliates, so the ratings did not return to previous levels.
Now CBS was in panic mode. In the summer of 1978 they added the "star wheel" to choose the head-to-head Super Match. That irritated Richard Dawson, who had been the favorite choice of contestants about 90% (or more, depending on whose version of the story you read) of the time and with that spotlight essentially removed, he left MG a few weeks after the wheel's introduction to concentrate on FF, which turned out to be a shrewd move on his part. Sidebar: The angular blue and white set was introduced right about the same time as Dawson's exit.
The episode you saw this morning on GSN was close to the end, as it did not last very long past changing the "78" to "79" on New Year's Day. The last MG 79 aired April 20 of that year, but a daily syndicated daytime edition debuted that September, running for two more seasons. The evening Match Game PM version, which had debuted in 1975, was still more popular than the syndicated daytime version, but lost affiliates its last two years because of most stations choosing one over the other; only in a few markets were both seen, such as New York where WABC-TV carried both.
MG PM ended in 1981 after six years, the syndicated daytime MG ran for exactly three years and ended in 1982.
And the decline in celebrity contestants was in lockstep with the falling ratings. My supposition is that if CBS had left it where it was in late afternoons it probably would have survived affiliate pre-emptions for longer than it did, because they likely would not have felt the need to add the "star wheel" and Dawson would have happily done both shows as long as Goodson-Todman would pay him to. The question would have been how long CBS would have been content to spotlight in that manner the host of a game show airing on ABC.