Jeff Atwood's answer has given me a new idea on the topic.
The problem with OnStartups was insufficient moderation, so that the site being open for public would risk drowning in crap.
If so, instead of deleting the site, why not keep it, but close it to new users.
- allow posting questions only if you have earned at least 25 reputation on the site
- allow answering and suggesting edits only if you have at least 100 reputation on the site (which means, you are either old user or you have association bonus).
This would strongly reduce the amount of crap, and therefore the moderation needed (not eliminating it completely, however).
Yes, it would require additional development work, but the topic like startups is quite attractive for Google and could bring some money from ads, so deleting the site is throwing baby out with the bathwater, a disadvantage both for SE and for community.