I don't know about not being able to run 15 years down the road. When the cars get older, all the bugs are fixed. So by the end of the life cyle, the car should be more or less as good as it gets. You won't need to update anything really. On the other hand, if your hardware breaks, then it will be costly.
Plus, refusing to get an M5 now means that you'll be that far down the waiting list... which is a very long list
