Endless punishment is the result of ENDLESS sinning.
My two cents
Some people say that eternal punishment (whether it be conscious punishment or annihilation) is unfair. But, when you decide that something is unfair, you’re making a value judgement over God. I don’t recall people were given that opportunity—Christianity is not about worshiping God just because you happen to like him. He precedes you, and the idea is to follow him on his terms, not for him to follow you on your terms (which would be humanism).
What shall we say then? Is there unrighteousness with God? God forbid. (Romans 9:14).
Boettner, L (1968). The Reformed Doctrine of Predestination. Philadelphia: The Presbyterian and Reformed Publishing Company. p. 79. (emphasis in original)