I does seem so blindingly obvious when you look at it that way.
It’s just a big house of cards. Pull the bottom one out (Adam and Eve) and there’s nothing supporting the rest of it.
Christians will often tell you that the Old Testament is “old” and really not necessary for the truth of Jesus. It’s frustrating when they want the country to follow the Ten Commandments, but when you point out to them that the Commandments are a bit archaic for the modern world, or even for the world of Jesus, that they really are not the basis of any real morality, they tell you that those were meant for the Jews, and we shouldn’t be so literal in interpreting them. Same thing if they cite Leviticus to justify their homophobia, and we point out that they don’t also observe the Levitican requirements about stoning, and mixed fibers and shell fish. “Oh. Those are old Hebraic laws and don’t apply to us”.
But original sin comes from Adam and Eve, and if Adam and Eve didn’t exist, then Original Sin didn’t exist, and if Original Sin didn’t exist, then sending Jesus to die to save us from it makes no sense, and is pointless!