It's a novel coronavirus for which we have no vaccine, few accurate antibody tests, a 60% confidence level for swab tests to find people that are largely asymptomatic, passing on the virus to others before they themselves have symptoms, and killing off roughly 1-2% of infected patients.
We have no idea whether it poses a risk to children and working-age people or not. In the short-term there seems to be no lingering issues. Long-term? No idea.
This coronavirus is roughly ten times as deadly as the flu (1% mortality compared to 0.1%), and it spreads roughly three times faster.
Imagine if there's immunity but only for six months to a year for most people who catch it. Imagine if the vaccine only covers expected popular strains for that region, but a new strain infects the region and results in an epidemic.
This would be on top of yearly flu outbreaks and it would only add to the chaos of a swine flu outbreak if that were to get out of hand.
Think of what that might look like. Flu deaths in the US were expected to be between 40k and 75k this year. COVID-19 outbreaks, if not caught early, could match that.
And it goes all around the globe in less than a month. One country not managing their outbreak properly puts the entire planet at risk of another global pandemic.
The worst thing for the economy would be not acting at all to prevent disease spread, followed by too short a lockdown, according to research based on US data.
phys.org
The linked paper is 56 pages long and goes through a lot of scenarios including herd immunity.