If the lockdown is ineffective, there's not point in doing it so whatever.
If the lockdown is effective, you kill the exponential spread. This is good, but if there's no exponential spread, you haven't actually infected many people at all. When you lift the lockdown, you're close to the same level of vulnerability as when you began. For a virus like this, you likely cannot 'eradicate it' by keeping the lockdown ongoing.
There's no middle ground where you slowly build up immunity across the population at a level hospitals can support. We can't turn the economy off and on again until the virus has had burst spread either.
The lockdown makes a lot of sense for 3 reasons: to learn more about the disease, to gather more PPE for doctors, and to reformat critical systems to run with a healthier population. It's not a bad idea. But it's utility is going to run lower while the cost gets higher. We need to address that.
The point of the lock down is to prevent the spread of the virus from happening too fast and overwhelming the healthcare system. It is not to prevent people from ever getting it as that's clearly not possible. You seem to be missing that point in your argument.