I don't see the point of the sentence. Please help!

Hello everyone. I really need your help. I've been reading that paragraph a million times, but still don't understand. What does the author want to say? Why do the lines converge over time? I don't see the point. What's the point of that paragraph? Please 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...'

'...There is another important thing to notice; the two lines converge over time. When David Mirkin had to replace half the writing staff at the beginning of Season 5, he brought in a number of writers who stuck around for three or four seasons, through the administration of Bill Oakley and Josh Weinstein. Those people also began departing after Seasons 7 and 8, which is why the lines converge as you move into double digit seasons. In effect, the Mirkin-hired core of writers lasted about as long as the original Simon-hired contingent, three or four seasons...'