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.
Just switch it to opt out, not opt in. In Australia you can opt in to organ donation but many people don’t care either way. My partner would definitely opt out because they don’t feel OK with it, and fair enough, but most people actually don’t care and would go with the default.