When a person sacrificed a lamb in the Old Testament, he did lose that lamb permanently, but if he had a healthy flock that lamb was replaced very soon. The point of that object lesson was the undeserved nature of the death, not the permancy of the death.
The lamb did not deserve to die, yet it was killed. Christ did nothing to deserve death, yet he was killed. The sacrifice was an undeserved death.
Humans can die for their own sins, but afterwards face eternity seperated from God. Or, we can accept God’s proposal. He sent Christ to die for us. If we agree with God that we deserve death and accept Christ’s death and resurection for us, than we will not spend eternity seperated from God.
That is the deal. We can accept it or reject it.