Could you please help me understand the first sentence of the last paragraph?

Please help me understand the first sentence of the last paragraph or make that more clear.

The Simpsons went through four different show runners during its first eight seasons. The first two seasons were run by Sam Simon, a long-time television writer who assembled the original writing staff. Seasons 3 and 4 were jointly run by Al Jean and Mike Reiss, who were among Simon’s first hires. Seasons 5 and 6 were run by David Mirkin, who was brought in when Simon’s original staff began to depart; and Seasons 7 and 8 were run by Bill Oakley and Josh Weinstein, who came aboard in Season 3.

The original Season 1 writing assignments are credited to just a few guys: Sam Simon, Al Jean, Mike Reiss, Jay Kogen, Wallace Wolodarsky, George Meyer, Jon Vitti, and John Swartzwelder. As the show took off, two more regular writers were added for Season 2: Jeff Martin (who is credited with many of the show’s famous songs) and David M. Stern. With plenty of help from outside writers who penned the occasional episode, that core of guys wrote the first three seasons of The Simpsons and created all but a couple of the secondary characters that have given the show such breadth.

Starting at the end of Season 3 and moving into Season 4, the writing staff got more fresh blood in the form of Bill Oakley, Josh Weinstein, Conan O’Brien, Frank Mula and Dan McGrath. But Season 4 also saw the first significant departures. After four years, Simon and many of the people he’d hired were burned up, pissed off, or simply wanted out. Kogen, Wolodarsky and Stern’s names stop appearing in the credits in the middle of Season 4. Jon Vitti and Jeff Martin departed at the end of that same year.

That was the first big change in the writing staff, but the show itself didn’t skip a beat. Several of the replacement writers had already been with the show for a season and the overall staff still had a strong and experienced core. David Mirkin, the new show runner for Season 5, built around that core and expanded the staff significantly. Remarkably, several of the people he brought on board, notably David S. Cohen and Greg Daniels, were writers on their way to the top of the television world. In terms of people who would later go on to huge success at other programs, the show peaked during Mirkin’s tenure in Seasons 5 and 6.

But all good things must come to an end, and that includes the writing staff of The Simpsons. When Bill Oakley and Josh Weinstein took over from David Mirkin at the start of Season 7, they had plenty of Mirkin veterans, but they were among the last links to the early days of the show. Besides the two of them, the only other pre-Mirkin veterans were the Four Horsemen of Season 1: Al Jean, Mike Reiss, George Meyer, and John Swartzwelder. 

There are twelve other writers listed in the credits for Season 7, which means that less than a third of the staff had been around prior to Season 5. Al Jean and Mike Reiss left after Season 7, as did future television comedy superstar Greg Daniels. The departures continued through Season 8 and Season 9, when Oakley and Weinstein were replaced as show runners by Mike Scully, who had come aboard in Season 5.