Time investment is not the same thing as difficulty.
The idea is to complete the fight within a time limit using a restricted set of resources that regenerate at a set rate. There wouldn't be much challenge to it if the encounter could be defeated one tiny section at a time, pausing the timer to let resources completely regenerate before continuing to the next tiny section.
If the group was then challenged to do the entire encounter without stopping after completing it one section at a time, they would fail because they wouldn't have worked out their resource management. Proper resource management is only going to be learned by repeating from the start so you'll know what's available at what points in the fight, what is best used earlier and what is best saved for later.
Having to restart a fight doesn't make it more difficult. Technically the difficulty remains the same. If anything, each new attempt should be just a little bit easier because you've mastered earlier parts.
If willpower becomes an issue, it's because your group is pushing itself harder than it should. Or because some or all of the group won't acknowledge that it's a challenge that just isn't enjoyable to them.
If you're not having fun, don't do it. Developers watch what players do far more closely than they listen to what players say when it comes to making changes or creating future content designs. If large segments of the player base keep forcing themselves to do what they don't like, they're going to get a whole bunch of the same in the future.