As I understand it, it’s not the paychecks for the acting work, it’s the constant cash flow from the syndication of the show. Maybe “set for life” can be overstating it for your average series, but for shows like “Star Trek” or “Seinfeld” or “The Office”, those folks will likely be seeing rather decent checks for the rest of their lives.
It all depends on whether you’re a “profit participant” in the show, which gives you a piece of the syndication revenue. On NCIS, for example, Mark Harmon is clearly in that category, but I’d be surprised if the rest of the cast has a cut of syndication profits. William Peterson of the original CSI left after nine seasons but still got paid as a producer, which gave him a share of the syndication profits as well. He’ll make far more off syndication deals for CSI than he ever earned as an actor (and he pulled down over $500,000 a episode at the end of his run).
I’ve read countless stories about supporting actors getting huge paychecks during the last couple of years of a series. In virtually every case, they’re getting their money up front, and will earn little/nothing from syndication and marketing. That’s one reason that virtually every writer on a TV show is also billed as a producer. It not only bumps up their weekly check, it also gets them into the syndication money.