We may have miscommunicated. I'm fine with making the box-ticking mandatory, but not the donation mandatory. In your earlier messages you asked about mandatory

We may have miscommunicated. I'm fine with making the box-ticking mandatory, but not the donation mandatory. In your earlier messages you asked about mandatory donation, not mandatory box-ticking. Opt-in-by-default does have the disadvantage you mentioned, but I don't think it carries much weight. Society could solve this problem just by sending a certificate/card to people who opt-out to tell both them and medical practitioners of their choice.