As far as I know, COVID isn't going to stop spreading around the rest of the world. So as soon as the lockdown in a given country ends, that country will be exposed to it and face the same pandemic they would have faced in the first place. Now, that makes a lot of sense to me if they are facing it with a new tool in hand (like the vaccine), that means a lot fewer people will die than if they'd gone through it with nothing.
But if there is no better treatment coming, what does locking down for 3 months, 6 months, or a year accomplish?