Except that the kernel is not designed, it evolves. Linus has pointed it out several times. At most, some features are initially designed, but from there on, it's all evolution.
Maybe you write perfectly designed code that works wonderfully and will never need an update, fix or improvement. I centainly don't. I make mistakes, that's why my code is full of assertions, and I try to manage impossible situations in a sane way, just in case.
With regards to fat and slow... Any check can be made optional, by simply putting it between a pair of #ifdefs. Maybe somebody will apreciate a more rugged, even if slower kernel for certain servers that are exposed to the outside World. An important point would be to measure how much slower that kernel would be.