Re: Valid policy
Plus there's the fact we now know the virus can be transmitted asymptomatically (a la Typhoid Mary), so as mentioned, there's no way to know just who's got it if they're not showing (or they're showing so mildly they're being mistaken for the common cold coronavirus).
So the problem with the scenario is that the unknown vulnerable are (1) much more numerous, and (2) impossible to actually determine due to the specter of asymptomatic transmission; it's a lot simpler (and not too far off) to just assume everyone is potentially vulnerable.