If TMP had delivered then things would have been different afterward. But there are reasons the film came out as it did and it seems no one would acknowledge what those were and accept responsibility. I suppose it wasn't unexpected TPTB reacted as they did by wanting to jettison everything but the cast, the sets and the filming miniature.
TMP echoed "The Cage" in some respects because it proved that Trek on film could work and make money, but it wasn't exactly what everyone had expected. All manner of reasons have been given, but if the film hadn't been saddled with the costs of the aborted Phase II production then the financial aspect looks a lot better because no one can say TMP didn't make money. It ended up being one of the better performing films.
Leading up to TMP no one could decide what kind of film they wanted yet everyone seemed to know what they didn't want. It's almost like "they wanted it all" and couldn't figure out how to get it. Hence the pressure to tell a "big" story. Although you can't blame the fans there was also a heightened expectation and with every fan possible hoping for something different from the next fan.
I applaud them for wanting to do something different from Star Wars.
It is more in keeping with TOS' somewhat more "cerebral" or thoughtful approach overall.
But the story they went with simply wasn't complete with all the kinks worked out before they switched gears and tried to rush out a feature film based on it.