Humans have decreasing marginal utility with money, so even if we model donors as perfectly rational (a bad assumption, of course), it still makes sense to donate some but not all of your money. Just because people sometimes have a bias towards finding a middle ground doesn't mean that finding a middle ground is always wrong every time.

The xkcd you linked to is a good demonstration of one of the reasons that asking people to go into debt to fund MIRI is a horrible idea.