[BananaRepublic]

It's not really about the 'development effort', it's about the performance. Message passing will always be alot slower than communicating through shared memory. Just like user-space filesystems will always be slower than kernel-space filesystems.

This means that in areas where performance is paramount, the system stability benefits of a microkernel design just isn't worth the loss in speed. In other areas it might be, but as the current OS landscape shows it's not in huge demand. Part of this is obviously that monolithic/hybrid kernels really aren't that prone to crashing (bugs get fixed), which means that in practice these kernel designs offers BOTH performance and stability.