Take for example TNG or Voyager. Basically their best work was season 1/2 and the last season. Why? In the first season they were trying it out. Seeing what was cool. The middle seasons are meandering and rather boring. The last seasons though it was more 'screw it we are not getting renewed lets do something interesting'
With Voyager's first two seaons, to me it felt like they were trying it out but never finding a good foundation for the rest of the series. There was only one really memorable character (the doctor) and the antagonists (Kazons) were uninspired. So in season 3/4 they gave the doctor more screen time, brought in the Borg and added a new character (Seven of Nine). You could call that pandering to their audience, but it did improve things a bit in my opinion.
Still, for me Voyager had a good premise but failed to do much with it. Perhaps because they went to one-off episodes pretty quickly, while the "in hostile space a long way from home" theme would have worked better with more continuity. Compare it to for example the first two seasons of the Battlestar Galactica reboot: even a simple detail like the population counter they show every episode does a lot to reenforce that theme.
Speaking of the Battlestar Galactica reboot, there the first two seasons are also the best, but my guess is that's for exactly the opposite reason as the one you named: in the first seasons it feels like they knew exactly what they were doing, while in later seasons they were trying things out and losing focus. I wouldn't be surprised if they started filming with 2 seasons of fleshed out scripts and had to write the rest as the series was already running..