In your scenario, borrowers would default, the risk premium for the remaining bonds would skyrocket, and bonds would also be close to worthless. Meanwhile, I would collect the $500 a month on my unleveraged properties and be just fine.
ETA: Reserves are cash. Stored in FDIC insured accounts and US Treasuries. If they go under, I don't think the paper asset markets will be functioning any longer.
I have plenty of cash reserves as well. Even during the down turn I didn't have any problem finding quality renters so no loss in rent. Being as my area does get hurricane scares it's possible I could sustain damages at multiple homes but not likely all of them at one time. And they are of course insured. So there are risks but the stock market could crash significantly again too and that's not insured. I've had rental houses for going on 35 years now, several have paid for themselves several times over, so even if they were gone the land alone is worth what I originally paid for them. It is important to maintain adequate reserves to cover the what ifs of course.