I would agree with you that attempting a startup is a net positive career, and even life, experience.
I would disagree that you should grind away for it on years. If YC is confident that hustle/effort is the problem, they should be hiring engineers directly, pay them market rate, and keep all of the equity in their projects to themselves instead of having founders shoulder the risk.
I think a lot of your replies and central argument is very thoughtful, but the one thing to consider is that successful startups are the result of unreasonable exertions of will, unreasonable amounts of luck, and an unreasonable amount of risk.
If it were reasonable, I don't think the value created would be so great.
While I don't disagree with your points on a macro, startups are very specific entities, and their success is contingent upon founders (and team members) who are willing to do unreasonable things. And also be extremely lucky.
I would disagree that you should grind away for it on years. If YC is confident that hustle/effort is the problem, they should be hiring engineers directly, pay them market rate, and keep all of the equity in their projects to themselves instead of having founders shoulder the risk.