The problem is verification. As things are today, you have to verify that a person has registered as a donor. If they are and you can't confirm it then nothing happens as far as that person is concerned. In the new system if you can't confirm someone is a non-donor they'll take all your usable parts.

I propose a compromise where every person is "supposed" to register their preference one way or the other. That would probably increase the number of registered donors, and if you can't find someone in the database you don't assume a default choice for them.