If a retroviral like Favipiravir, currently on clinical trials, is proven to be effective, then patients can be treated before COVID-19 fucks up their lungs.
The lockdown measures have a threefold effect:
1. Flatten the curve so that patients trickle into hospitals rather than flooding them, this in itself prevents hospital system collapse and lowers mortality.
2. Slow down the spread of the virus which again prevents deaths and gives governments more time to:
2a. Come up with an effective treatment
2b. Get more respirators and build more hospitals
...thereby preventing even more deaths.
It's not gonna collapse society either way. You could theoretically let it rip and allow it to kill millions, society would still go on, but the cost on lives, and probably the long term economic cost as well, would be greater.
The earlier lockdowns are imposed, the faster the spread slows down and the more cost-effective they are. Countries that adopted lockdown measures later on, have much more cases, and thus will need to stay in lockdown even longer for any significant gains to show.