I agree that US Air's behavior appears outrageous based on the OP.
(I don't really understand how a previous poster is unable to distinguish between one time denial of access to a club and permanently revoking something that the OP paid good money for.)
However, I can imagine one thing that US Air might suspect happened. Suppose OP appears to be a young man in his 20s and has recently begun presenting a long dormant lifetime membership card. Could the club be concerned that really belongs to his same-name father rather than him? That would explain why US Air thought there's no way the OP could have a lifetime membership.
A quick check of the OP's driver's license would determine how old he was in the 80's. And I don't think that a lounge agent should have the power to revoke someone's lifetime membership. After all, if they are still honoring lifetime memberships pre-merger, then the OP was not in the wrong. It's irrelevant how much he paid for it. If they don't think it is his, then who do they think it belongs to? If they think it was his father's, then a copy of his birth certificate should show what his father's name is.