In regards to how much I would need to stay in business: The current cost to keep the site up is just under $200 per month. My revenues are already above that. Obviously my credit card transaction fees need to be deducted and the registrar I resell through will get a cut. I plan on paying that out of pocket if it makes no money because I enjoy using it for my domain portfolio and I think my friends do too.
In regards to what would happen if it did shutdown (I obviously would prefer this not to happen): The registrar I resell through (Key-Systems) would then be the registrar you would use. They are among the 15 largest registrars (according to ICANN) and manage more than 3 million domains.
I hope that answers some of your questions. I do agree that with something like domain registrations you have to be very careful.
Thanks!