We do not need our bodies once we leave this world regardless of what you think happens after we die. We should be focused on curing diseases and extending the life of living humans. Science would go so far if we used human bodies after death instead of requiring people to give consent to something they don’t need.
Why don’t they require people to go to a government building and sign paperwork to get off then get it added to your state ID that can be scanned at death to tell if you’re an opt out