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.
Organ shortages are definitely not covered by opt in.
Opt out also doesn’t mean that scientist have to use all the bodies.
It’s never been a problem to have too many bodies. It’s a lot easier to turn away some when they’re not needed than it is to find one when needed.