I must make a brief detour to psychology. In particular, I must explain Brehm's motivational intensity theory, which has not only
been tested in video games and held up, but overall has been replicated so extensively that
Richter, Gendolla, and Wright's summary paper had to direct readers to narrower summary papers. For brevity and clarity despite machine translation, I will explain only the points of the theory that relate to the question of balance at hand, and I will explain via example.
Suppose a player is considering a goal with a fixed difficulty, such as completing a dungeon. If they had to do something ridiculous to accomplish that goal, like find three other people and teach them the game in order to have party members, the player would almost certainly reject that goal and find something else to do with their time. Likewise if they had to do something blatantly impossible. This illustrates that they have a maximum amount of effort that they find worth it or possible, which we call their "potential motivation" in the context of that goal.
However, even if the player can complete the dungeon within their potential motivation, that does not mean that they will put forth that maximum amount of effort - they may be willing to spend an hour in the instance, but if the party completes the dungeon in twenty minutes, they will not then spend the remaining forty minutes in the same instance pressing their buttons with no targets. (They may well spend it in the same instance exploring the environment, but that is a new goal.) Rather, they will put forth the minimum amount of effort needed to accomplish the goal, and no more than that.
This theory makes intuitive sense from the perspective of conserving energy. A rice farmer who insists upon moving to a desert and constructing new rice paddies there will probably produce no rice and starve. A rice farmer who insists on hauling their harvest back and forth for no reason, not even physical training, will not put themselves in a good position either. The many experiments confirming the theory show that it is very much correct - conserving energy in this way is an evolved imperative of the human brain, and games for humans must bend to it.