How often do you want to do that worldwide travel shutdown? We got through SARS and Ebola without one. At the height of each, without the benefit of hindsight, did you consider them less serious then this one?
I have advocated a worldwide travel shutdown exactly twice in my life: SARS, and now. SARS was a damn close call; it was stopped by proactive measures of contact tracing and quarantine (i.e. you quarantine, forcibly if need be, not only each sick person, but everyone they came into contact with); had it not been stopped, the death toll would've hit nine digits. This one looks about equally serious, and has clearly not been stopped.