What could possibly prevent me from signing a fake video with a cryptographic key based off of scan of your biomarker, once I obtain that scan somehow ? Biomark

What could possibly prevent me from signing a fake video with a cryptographic key based off of scan of your biomarker, once I obtain that scan somehow ? Biomarkers can't be changed or revoked, so once they leak, you either have to change the whole system or stop trusting your content forever. (the details of obtaining the marker aren't relevant, I think you'll agree that one way or another there will be some leaks or cracked phones or any one of other possibilities eventually exposing biomarkers for at least some of the population)