Let's just say I am a lot less idealistic about colleges than you appear to be.
The top students who will become prestigious alumni are by and large not the same students who don't know what they want to major in yet. For every Steve Jobs-like outlier you have a ton of executives, accountants, doctors, and engineers who know that's what they want to do. The major-less students will generally either pay for four years of psychology or be successful for unrelated reasons (legacy status and family connections, the occasional dumb luck).
If YC thinks trying to pick out the outliers, the lucky, and the connected from their application pool is worth the trial, power to them.
The top students who will become prestigious alumni are by and large not the same students who don't know what they want to major in yet. For every Steve Jobs-like outlier you have a ton of executives, accountants, doctors, and engineers who know that's what they want to do. The major-less students will generally either pay for four years of psychology or be successful for unrelated reasons (legacy status and family connections, the occasional dumb luck).
If YC thinks trying to pick out the outliers, the lucky, and the connected from their application pool is worth the trial, power to them.