When beleagured CBS News president Andrew Heyward finally resigned last Friday, many wondered what had taken him so long. Ever since 60 Minutes’ botched 2004 story on Bush’s National Guard service resulted in the purge of star producer Mary Mapes and three other staffers last year, Heyward’s endangered status at CBS had been painfully apparent to network insiders. (CBS kingpin Les Moonves made little effort to conceal his disdain, complaining to the New York Times about Heyward’s failure to “lead a revolution” in the perenially trailing news division.) But in the end, sources say, it wasn’t pride or professionalism that kept the embattled exec hanging on at Black Rock. It was the lucrative pension that kicked in on October 29, his 55th birthday.

The milestone, which we hear made him eligible for a stipend hefty enough to keep him smiling the rest of his days, was no secret to the journalists at the heart of the Memogate scandal. “Heyward will not last a day past his 55th birthday,” Dan Rather told a friend not long after the network launched a probe into the sourcing of the (ultimately retracted) exposé.

“This guy could have been a hero,” says a source close to Heyward. “He could have told Moonves and Sumner Redstone [the head of CBS’s parent company, Viacom] that they would have to fire him before anyone else on his staff. Instead, he took the coward’s way out and let them fire Mapes and destroy the good reputations of three other journalists.”

Mapes herself doesn’t mince words about Heyward in her new book, Going Down, which was released yesterday. “He did not protect and defend his department or his employees,” she writes. “Instead, when the going got tough, this captain of the vessel got tough, too. He made sure he survived. I’m afraid if Andrew Heyward had been on the Titanic, he would have been among the handful of men said to have slipped on long gowns and elaborate veiled hats and shoved their way to the lifeboats.”

Some speculate that Heyward, who worked at CBS for more than 24 years, was allowed to reach the benchmark due to his special relationship with Moonves’s wife, Julie Chen, who rose quickly through the ranks of his news division to land a co-anchor slot on The Early Show. But while Heyward has said he’ll stay on as an adviser to newly promoted news president Sean McManus until his contract expires at the end of the year, it’s clear that there’s no love lost between Heyward and Moonves. In a bilious memo to his colleagues, Heyward hinted that his retirement was less than voluntary: “As for my own plans, I do not intend to ‘pursue other interests’ or even ‘spend more time with my family,’ wonderful as it is. I am going to remain fully engaged in the media business.”

Asked about the details of Heyward’s pension, Gil Schwartz, a spokesman for the network, delivered a profanity-riddled lecture on journalistic propriety before declining to comment on the record. Another rep for the network, Sandy Genelius, also refused to discuss the pension deal, but described Heyward’s departure as “the most ordinary and respectful transition I’ve ever seen.” Rather, who is traveling, could not be reached by press time, and Heyward did not return calls.

* Due to a labeling error at our photo agency, a picture of a similarly mustachioed Memogate casualty—former 60 Minutes Wednesday executive producer Josh Howard—briefly appeared on the site this morning in place of Andrew Heyward. Apologies to Mr. Howard.