Internet commenters are right, Piëch should have included in his briefing to his engineers, “Make it easier to service than a Golf for that one customer who won’t pay the reasonable service cost.”
But there are reasonable and knowledgeable people in the comments. Apparently the annual service includes removing the cylinder heads to make sure all the components are being lubricated properly. So $20,000 seems pretty damn reasonable for all of that work. But, on the other hand: CYLINDER HEADS OFF EVERY YEAR?!?!?
Oh and someone even mentioned AMGtech's post about the oil pan on the GT. It never ceases to amaze me how people honestly think a high performance car should be as easy to service as a Golf. It's almost like they don't understand how cars are made, or how to work on them.