Actors in successful television series earn much less today. My accountant does television acting work and says that the salaries are way down. His gigs are typically small roles and does the work mostly as a hobby during the slow accounting months, but he says the big stars earn a fraction of what they did earlier.
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.