>If you have a single well reviewed implemention I think you are much better off than having 10 badly reviewed ones.
That sounds good in itself. However, your advocated position is to standardize on a kernel that's featureful and consequently full of bugs (despite having so many developers contributing to it), to the complete exclusion of any kernel with fewer bugs. That's not to say that your advocated position is a bad one; but I do believe it runs counter to a goal of combatting computer insecurity.
(Incidentally, if you really did think that 10× the number of bugs is such a bad thing, then I think you'd probably use a different kernel. But most people do use Linux even if they know it has lots of bugs, and would even if they knew it had 10× the number of bugs of some other unixy kernel, even for just one or two extra features important to them. Similarly, each of the N competing implementations presumably has one or two features or attributes not present in (and not feasible to add to) the others.)