The first part of your post complains about how few YC partners have led a product to a meaningful exit, but the last part complains about how they lack experience building a long-standing business. The two of them are to some extent mutually exclusive: once you exit, you are no longer building a long-standing business. (There are some exceptions where a company is acquired and then continues growing under the acquirer, like YouTube, Instagram, Twitch, and Reddit...and founders of the latter two happen to be YC partners.) Which do you care more about?

For building a long-standing product or business, I would argue that Pinterest, AirBnB, 23andMe, GMail, and Yahoo Mail all qualify, as does YC itself. That's why they haven't exited: Ben/Joe/Anne are all still building their businesses.

For exiting: PG/TLB/RTM (Viaweb), Kevin Hale (Wufoo), Justin Kan (Twitch), Michael Siebel (SocialCam), Paul Buchheit (FriendFeed) and Alexis Ohanian (Reddit) all exited for F-U amounts of money. Sam Altman (Loopt), Dalton Caldwell (iMeem), Garry Tan (Posterous), Qasar Younis (Talkbin), and Geoff Ralston (Lala) may have, but it's unclear from public documents whether the acquisitions of their companies resulted in a significant payoff to them after the liquidation preferences of their investors are taken into account.