They're going to die anyway and get to contribute to scientific progress and get the best chance possible today for immortality.
They could contribute to scientific progress by donating the money to a research university. Depending on your concept of immortality donating sperm or eggs seems to have a higher chance of success.
It's not a guaranteed win, no. However, if they are at the point where they wish to be euthanized then their outlook is grim. However, even a minuscule chance is better than zero chance, correct? In addition, by donating their brain structure they're directly fueling the research needed to improve that chance.
They certainly don't lose. They are all to be patients who are going soon anyways, who opt for the process.