For full details read Bill Carter's "The Late Shift." It isn't in print anymore, but copies can be found easily on Amazon, eBay, etc.
By the time the 90s rolled around NBC execs were starting to feel that Carson was too old. But because Johnny was still so big, NBC wouldn't touch him. Letterman had been doing Late Night for a decade and was waiting in the wings. He was Johnny's chosen successor and refused any contract that offered him The Tonight Show upon Carson's retirement because he didn't want to be seen as pushing Johnny out. Letterman idolized Johnny. But Jay Leno, Johnny's current guest host, wanted The Tonight Show and didn't want to have to wait for Carson to retire. So Leno's agent drafted a fake news story and had it published in The New York Post. The story proclaimed that Carson was out as host and Leno was in. NBC denied the story. But not very strongly, because secretly, it's what they really wanted. Johnny got pissed at the network and retired early.
So ended Johnny Carson's career. Not because he was ready to retire, but because Leno's people planted fake stories in the press. It was all a backhanded move to get Carson out of the way so Leno could take over. Then he only had to screw Letterman out of the job. Which he did. Leno hid in closets to eavesdrop on NBC meetings and his manager browbeat the execs (most notably Warren Littlefield) into giving the job to Leno, instead of Johnny's guy, Letterman.
I always thought, by reputation, that Carson was bulletproof too. He was powerful, but not so powerful that a fake news leak couldn't get him out of the way. Leno did that. That's what the 17 I'm talking about.