Hate to even invite the rabbit hole, but even on HN you will find many people who fundamentally disagree with your assumption that copyright itself is reasonable in the first place.
It is akin to the marijuana legalization movement in many ways - pot was always illegal, so it should always be illegal, and to question why it is illegal is to be unreasonable. Debate about sentencing, classification, possible medical use - possibly reasonable, but to question the fundamentals starts drawing on cultural and emotional ire and identity that starts breaking down reasonable debate.
Copyright to many works much the same way - especially when no new works have entered the public domain in the lifetime of the vast majority of the populace - it has always been there, thus even if you debate duration you assume it must remain because to question why it exists in the first place is, again, to be unreasonable.