Thanks for the reply, Marco.
Suicide would offer no benefit to anyone: the payment would be made at the time of the binding agreement to donate your organs; the completion of the contract would happen after the death.
The problem with opt-out is the fact that implied, tacit agreement is morally questionable. Imagine there was a similar arrangement relating the destination of your assets after you die. The default is that it all goes to the state; if you disagree, you simply need to register your objection, and only then will your assets go to your heirs. Is that morally and politically acceptable?