I know nothing about all of this, but I guess the idea is there are too many closely concentrated people and too many variables to control for, so if there are

I know nothing about all of this, but I guess the idea is there are too many closely concentrated people and too many variables to control for, so if there are new cases, they could potentially turn into new outbreaks if they aren't detected and handled in time. And it's likely just a matter of probability whether or not they all get detected. Even the most advanced and restrictive surveillance state in the world can't possibly catch everything with 100% probability. I think there's a decent chance the outcome won't be nearly as bad as the worst case scenario, but the worst case scenario is pretty scary. There is a non-zero probability that this wave is only beginning, that there are going to be future outbreaks and pandemic waves this year, and that by year's end there'll be millions of deaths globally.