The entire TNG cast could have been replaced
The first two to three seasons of Star Trek: The Next Generation were rocky with writers abandoning ship and/or being fired, struggles for powers, and dissension that often caused behind-the-scenes battles. Maurice Hurley, the showrunner most notable for letting Gates McFadden go at the end of the first season, was calling the shots. Things were getting so tense that Hurley told Rick Berman to fire the entire cast at the end of the first season.
Hurley told William Shatner in William Shatner Presents: Chaos on the Bridge that he would have built the second season around finding a new Enterprise crew after the original Enterprise with Captain Picard, Commander Riker, and the rest of the crew exploded for some unknown reason. Everybody would have been lost, giving Hurley free rein to start anew.
Could TNG have survived with a new cast?
There were doubts that TNG would survive its first season when the series was first created. Not every one of the episodes had the greatest story lines, and some were just downright abysmal. But what did work was the cast. Patrick Stewart, Jonathan Frakes, Brent Spiner, LeVar Burton, Michael Dorn, Gates McFadden, and Marina Sirtis all connected, and that’s what kept the fans coming back. They believed the show would get better. And it did.
Starting from scratch with a brand-new cast could have very well been the death knell for The Next Generation as well as for future Star Trek shows. To have the wonderful cast ripped away after we’d barely gotten to know them wouldn’t have been a good thing. And, personally, I don’t think fans would have returned for the second season because they wouldn’t have been able to trust that the new characters wouldn’t have perished at the whim of the showrunner.