How is it YC's problem? The founder is choosing to start a company and asking YC for money. Clearly they're not short of startups on the supply side of their equation.

You're not thinking about it the right way. It doesn't matter if they have "enough" applicants. They're trying to fill the positions with the best candidates out there - who they will just have to compete for. The more barriers they put up, the shittier the candidates they will get.

Why would Y comb need to compete with someone who is considering an offer to G? In that case, the founder would leave for the better deal anyways - so it's best to leave before it gets REALLY hard.

Ok, but now you're filtering out a good chunk of people smart enough to work for G - so your applicant pool just got dumber.

Disagree. If someone's choice either is to work for G, or start the company, it is the founders risk to take and give up that job for the outsized risk and return of starting up. It is not YC's need to compensate a founder for their alternative.

If you think about these things in terms of "should" and "who's responsibility" you will never understand how it works.

What are the founder's other alternatives? What are there other early stage & incubation options? YC simply does have to compete with that.

I mean, look. Take your idea to the extreme - why do they pay a stipend at all? Why not force the founders to sleep in tubes above the office/outside on the street and supply just enough soylent, huel & low-dosage amphetamines to keep their brain working?