YC is clearly not necessary for survival, especially for someone who has already had an exit (and thus has some capital and contacts), or someone who is 50 and has run a 30 person lab, with some savings and other personal contacts. YC can probably help in both of those cases, but the company is capable of being successful without it.

I'd sure rather have an Elon Musk or a Mara Aspinall in YC than a randomly-selected person who already believes YC is the best (or only) way to do a startup.

There are two parts to this -- making YC as useful as possible to the best founders, and making sure the best founders know that and apply.