  First, as mentioned earlier, the amount of danger remains the same regardless of whether or not payment takes place. So if someone objects to paying donors on the grounds that payment will encourage excessive risk-taking, s/he ought to object to donation itself (including free donation) not just the payment. Imagine someone who objects to paying astronauts on the grounds that it encourages them to do something excessively dangerous. Surely we should say to her that, if the objection is danger, s/he should object to space travel in general, not (just) to paid space travel. Much the same goes for paying organ donors. If the worry is danger, we should object to dangerous donation of all kinds, not just the paid variety.