That was a major part of it, but the real problem was ABC kept making them make more shows (this was a time when you didn’t say no to any network)
eta: And my response to that always was like, “You are right. So let’s design a finite beginning, middle, and end.” ABC just didn’t want to engage in that conversation. At the time that they picked up the show, they said, “Make 13 of these, and let’s see how it goes.” It was such a ratings hit that it became clear to me instantly that all conversations about ending the show would be over. I said, “Hey, guys, we can’t keep this up forever,” and that’s when ABC said, “Oh, we were thinking more like ten seasons.” The compromise ended up being six, but I personally wish that we could have done it in four."