1. If very large numbers of people become ill, they drop out of the labour market while they (hopefully) recover. We can't say for sure precisely what proportion of Covid cases are truly asymptotic, but a reasonable guess based on countries like Iceland and Korea with very large testing programs is not more than half. This means that if Covid spreads uncontrolled, large numbers of people drop out of the workforce regardless. It also means that those involved in key professions (NHS, food distribution, water, gas and electricity supply etc etc) who will have to try and continue to work in any situation are more likely to become ill, as higher rates of infection in the general population mean any given individual, key worker or otherwise, is more likely to become ill.
2. Very high levels of spread of the Covid could well lead to panic. Lockdown or otherwise, people with savings or supplies might simply refuse to go to work, whether their employer allows them to or not. Those who are don't have the economic security to do this might just tell their employer they have Covid symptoms and need to self isolate.
3. Failure to enact suppression strategies will not be a politically sustainable option. If, as is likely, countries which stick to suppression strategies see a significant short term reduction in new cases and fatalities, any government which took a different route or reversed suppression measures prematurely due to 'economic' concerns would face extreme political pressure to introduce or reintroduce suppression strategies.
4. The faster and more thoroughly you enact suppression strategies, the more quickly case numbers fall and you can legitimately start to think about easing things off. Getting ahead of the curve sooner means not just lower fatalities in the short term, but a shorter period of economic disruption.
5. The long term economic damage from coronavirus isn't just about disruption this year. It's about what the labour market looks like after this is all over. Very large numbers of infections means, for reasons too grim and too obvious to spell out, a period of difficult adjustment in the labour market.
The broad thrust of these arguments is pretty simple. One way or other, we are going to see a period of major economic disruption. The choice is whether governments get ahead of the curve, and keep that period as short as possible, or let things get out of control and face the same, if not greater levels of disruption for a longer period of time.