[Are links to sci-hub ok here? 'Brain Death - Too Flawed to Endure, Too Ingrained to Abandon' by Robert D. Truog, Journal of Law, Medicine & Ethics, Summer 2007, https://sci-hub.se/https://doi.org/10.1111/j.1748-720X.2007....]
In my area, you can indicate that you want to be an organ donor on your drivers license. It's a check box. But can you fully understand your decision at the DMV office, and can you fully contain your wishes within a check box?
Similarly with the posted links: informed consent is important, even if that means fewer volunteers.