Post Merge, Why Do Government Agencies (IRS specifically) Have To Recognize ETH as the Original Token?

Just curious what the argument is. ETH is undergoing a massive change in its fundamental nature with the merge. ETHW retains that nature, as if the merge never even happened. While I definitely see from a crypto perspective why ETH remains the "true" chain and ETHW a "fork," can anyone construct an argument that suggests ETHW is legally less valid as the "original asset" than ETH, and that ETH post merge is not merely a windfall/airdrop token given that it represents something completely different? Our word or opinion as random internet crypto bros doesn't mean squat legally.

I'm curious because of potential tax and regulatory considerations.