Star Trek: The Next Generation is one of the most beloved shows of the franchise, so it's wild in hindsight to think that anyone would've missed out on a chance to get in on the ground floor. Of course, hindsight is 20/20, and there were many back then who questioned if another incarnation of the original series was a great idea. As such, it's not too surprising to hear Spock actor Leonard Nimoy passed up on a chance at being an executive producer, though his reason why isn't what fans may think.
An excerpt from The Fifty-Year Mission: The Next 25 Years: From The Next Generation to J. J. Abrams: The Complete, Uncensored, and Unauthorized Oral History of Star Trek by Mark A. Altman is making rounds on the internet, and features Leonard Nimoy's explanation for not taking a behind-the-scenes position for the Patrick Stewart-led series. Nimoy had his reasons but explained it wasn't for the reason some might assume (via ScreenRant):
Leonard Nimoy had a chance to be a significant contributor behind the scenes in Star Trek: The Next Generation, now available to stream with a Paramount+ subscription, but turned down the opportunity. As the actor noted above, this wasn't because he had mixed feelings about the show, even if he did question if it would be a success. In the end, it was more about not wanting to dedicate the next years of his life to the show and to pursue other things.
For context, Leonard Nimoy had just wrapped up Star Trek IV: The Voyage Home, in which he starred and directed. It was the fourth in a string of films done the years prior, so it's easy to understand why Nimoy might've had his fill of Trek at that point. Of course, he did do more movies for the franchise after The Next Generation premiered and even had guest roles on the series, so it's likely it was more tied to the time commitment.
This is fair, considering Leonard Nimoy went on a streak as a director following the fourth Star Trek movie. Had he served on The Next Generation as an EP, we might've never seen Three Men And A Baby or witnessed any of the other amazing things that make us love Leonard Nimoy. Being a guest star on TNG was enough and ultimately important for setting up a key part of the franchise's future with his role as a Romulan ambassador.
And while the late Leonard Nimoy is no longer with us, Spock's legacy lives on through others in upcoming Trek projects. The franchise continues to tell Spock stories through actors Zachary Quinto and Ethan Peck, though we're still waiting on word if Star Trek 4 is happening or not. Meanwhile, Star Trek: Strange New Worlds is heading into Season 3, with Peck asking some big questions about Spock. Of course, Nimoy's legacy as Spock is still well-celebrated, and his work is worth checking out on streaming for anyone who needs a reminder.
Those with Paramount+ can access a vast library of new and old Star Trek content. Those without can still get their fix by checking out Pluto TV's Star Trek channel, or checking out the newly added series Prodigy on Netflix.