I think too much hardship can wear us out, break our spirit, or disable us sometimes.
Here's a link:
http://jamesclear.com/willpower-decision-fatigue
https://www.psychologytoday.com/blog/the-science-willpower/200910/in-defense-good-nights-sleep
I suppose we'd have to look at bigger sample sizes to determine if it's a natural or learned characteristic. I know with my own kids, they are the opposite of what you suggest. My oldest, who was largely scheduled, doesn't have vast amounts of willpower. He never did... He was far more prone to meltdowns over homework and was more likely to try to find a sneaky way around waiting for something- i.e. steal cookies when I wasn't looking, hack the device that limited his video game time. My middle child and youngest were wholly unscheduled and attachment parented. They fed on demand, even if that meant I nursed them every 30 minutes, though there were obviously times where one would have to wait for one reason or another. My middle and youngest do fairly well with delayed gratification. My straight-A 8-year-old recently brought home an extra credit packet during the holiday weekend. When I asked her why, she said she didn't want to lose the #1 spot in class. I suppose I could argue that ensuring a child's needs and wants are met at an early age inspires confidence in them; that they know they will always get what they require, even if they do have to wait for it sometimes, whereas the "trained" child doesn't learn that waiting pays off... he or she learns that waiting serves no benefit i.e. that marshmallow in front of them is never really going to be theirs, so why bother to wait? Grab it quick while the teacher isn't looking. Babies don't understand clocks... they just know they're hungry and mom says no.
Eh... that is neither here nor there, I guess. Even if we did have a bigger sample size (and yes, a question is coming lol) the fact that parenting style may relate to a child's willpower doesn't necessarily mean one causes the other. It could be that the parents with more willpower are more prone to scheduling (or vice versa) and that they also pass down their super-powered willpower genes. Hmm... Curious, eh?