We can speculate all we want as to if WDW will shut down or not. It all boils down to one thing- if Disney's risk weighted cost of staying open exceeds the cost of shutting down the parks, they will shut down the parks. It really is as simple as that. Said risk weighted cost will include potential litigation, as well as reduced revenue from bad PR, among other things, times the probability that said risks will occur (i.e.- the risk of a person infecting others on park grounds, it getting traced back to WDW, and WDW being sued successfully). Disney, like all corporations of size, has numerous people (people trained in risk assessment, lawyers, accountants, likely even insurance actuary types) looking at these numbers, and I'm sure they are working overtime these days.
With 221 officially confirmed infected individuals in the US, that risk is extremely low (and yes, I believe the number of infected is actually higher than that, but these are the numbers Disney has to go on). If those numbers move up into the tens of thousands, and the 2-3% mortality rate, and R0 of greater than 2 holds, I believe the prospect of shutting down the parks would be in play. If there are hundreds of thousands infected with the 2-3% mortality and R0 of 2+, then I would say it is likely they shut down the parks. (this is all based on some simple back-of-the-envelope risk calculations, and I'm sure I have over simplified/missed some things, so take this with a grain of salt). However, at this point those are a lot of "ifs."