I think travel bans are largely pointless at this point, because people don't want to go to the worst affected areas and FCO advice is off putting enough.
The issue is finding a flight still going and hasn't been cancelled due to lack of demand, insurance and quarantine issues both domestically and abroad.
Number of cases isn't relevant here. Insurance companies will look at that but will also take into consideration the health care system of the country. The US with low testing and poor universal health care coupled with high health care costs is going to look a lot more risky than a lot of places. That includes places with poor health care systems. It'll probably be easier to get insurance for Vietnam for this reason.
Insurance companies will not want to take a hit on coronavirus related costs - especially if other health care systems are struggling.
Also insurance would cover your costs if you are involuntarily quarantined. This could be basic hotel costs but it could be much more expensive hotel, ambulance and health care costs as the US has been doing and then billing people for the privilege of this. Not all countries will go this far but its important to have an idea of which might be dodgy in this respect (clue: anywhere with universal health care is a much better bet).
A lot of policies do not cover you as it is. Some do but these will be harder to get hold of going forward.
So in terms of holidays and school trips that's where the rub is going to be. And many airlines / holiday providers won't offer refunds if your reason for not travelling is your inability to get insurance if your destination isn't on the FCO list of off limits places.
This is what people should be looking at.
Not the number of cases.