Gunsmoke was still doing pretty well, tied for 12th (but it was getting long in the tooth, and would come to the end of its long and celebrated run in 1974-75 [its 20th and final year]).
GUNSMOKE dipped to something like 35th place in Season 12 (high enough to keep most shows on the air, but notably low for GUNSMOKE) and the show was canceled for the second time in 1967; the cancellation decision was reversed (for the second time) and the show was moved from Saturday to Monday (thus causing the cancellation of GILLIGAN'S ISLAND to make way for GUNSMOKE).
GUNSMOKE rebounded almost immediately, despite being against LAUGH IN.
After Season 19, Miss Kitty left and the show survived it at first -- but when Leonard Katzman left midway through Season 20, ratings tumbled because the tone of the show shifted (i.e., too dark and ponderous, too much of the time) and the third and final cancellation happened in '75... Once again, the ratings were high enough to stay on the air. But CBS execs were after it because of aging demographics. ("Horsefeathers!," huffed James Arness).