Allow me to clarify/elaborate on my earlier post. Postulating a six year run for the original series, around the fifth year actor costs would begin to escalate. Network brass would conclude that they could keep the brand going if they launching a spin-off with a new cast. Such a spin-off would undoubtedly be set in the same timeframe because that would make more sense economically as props (phasers, communicators, etc.) could just be moved over. After six years of TOS and five of "Star Trek: Deep Space K8" or "Star Trek: Mariner" that would be ten years (assuming one year of overlap). Oh, we might eventually see something like a "Next Generation" series but that wouldn't come for a long time given that much more twenty-third century product out there in the syndication market.