The term growthers refers to the notion of exponential growth, and indeed the number of Covid-19 cases appears (by some accounts) to be following an exponential pattern. Some scientists have estimated that the number of cases doubles about every seven days. If you play that logic out, it is easy enough to see how people might be complacent at first, then in a few months there is a public health crisis.
Of course, that process of doubling won’t go on forever. At some point, the number of people who have already been exposed to Covid-19 would become so large that their immunity could lower the subsequent rate of spread. Furthermore, society would adjust by having fewer large gatherings — many conferences already are being canceled — and by taking other precautions.
Still, the growthers find it easy to imagine that the number of cases might overwhelm the capacity of the U.S. health care system. Even if you think a speedy American (or more likely Singaporean) response argues against this scenario, it is harder to be equally sanguine about all the world’s nations, most of which are much poorer and have lower-quality public health systems than the U.S.