I agree to this, mass immunity is what we need at the moment instead of complete shut downs. Even after shut down, the result isn't positive. The disease is mostly killing older peoples like above 70s, so the lockdown can be strictly on these people and other groups can be allowed to be in open so that they actually get infect with the virus and build the immunity since they won't die of it. However, those who are prone to the disease like elderlies or those with chronic diseases should be in a room quarantined so as to not get infected. This will be a far better approach I think.
Herd immunity without a vaccine is not a reliable option.
It's one we can use but not one we can trust. The trouble with viruses is they mutate all the time. A small mutation is fine and can be dealt with and often can be incompatible with the viruses survival but if it replicates enough then it can start to change how it functions and be a new strain that previous infect ants won't be immune to.
(the virus mutating all the time isn't new because every cell does it, it just may be new to you as a lot of replicas hinder the virus so the new strain doesn't survive as well).