Towards the end of the sixth season of Star Trek: The Next Generation, there were rumblings that Patrick Stewart would not be returning to the series. It was during "Chain of Command" that the writing staff heard that he might not return. They ended up bringing in a new captain in the first part of the two-parter, and for some, they considered a new captain would give the show an extra jolt that it would need for the seventh season, according to Captains' Logs The Unauthorized Complete Trek Voyages by Edward Gross and Mark A. Altman.
With the possibility the Enterprise might need a new captain, there were even thoughts of bringing in a woman and even "some enthusiasm" about choosing another actor to lead the crew. One member of the writing staff specified that they had more concerns about the potential for Brent Spiner to leave before the seventh season.
Calling Spiner "irreplaceable," they pointed out that they couldn't just bring in another android to take over for Lt. Commander Data, and their concerns were valid as Spiner has said previously that he thought about leaving the show many times. Fortunately, that didn't come to pass, and Data, as of the final season of Star Trek: Picard, is still around.
Jeri Taylor, the executive producer of The Next Generation's seventh season, admitted that the negotiations with Stewart for the final season were "difficult" and had he cosen to not come back, they would have left the door open for him. But she didn't think there was real concern about the possibility for a lengthy amount of time. Fortunately, Stewart did decide to stay on as Captain Picard, continuing for four movies and three seasons of Star Trek: Picard,
While the writing staff and others might think it would have worked to replace Stewart as the captain of the Enterprise, I don't believe that to be the case. It's understandable why they would have concerns about Spiner leaving, but losing Captain Picard would have affected the series as much (or even more than) losing Data.
Both actors were integral to the show, and their connection was also a major part of the success of The Next Generation. No, it wouldn't have been easy to find a replacement android, but finding an actor who could win over the audience as the captain for only one season would have been just as difficult. Patrick Stewart was Captain Picard, and to many, he was the only captain they'd known as they hadn't watched Star Trek: The Original Series. Replacing him with another captain would have probably been as successful as replacing William Shatner's Captain Kirk.