In a perfect wrold microkernel OSes would be perfect but then it's all pointless.
In real life there are certain parts of the OS that have to work or the whole device stops working. Furthermore: the isolation of dynamic and less tested application code from these parts is generally a good idea, that's why monolithic OSes are so popular; they're simply less demanding.
Nobody claims faults of core functions must be survivable in microkernel designs. The restarting of driver processes is a nice trick, but in the end the isolation and clean interfacing are particularly useful in an imperfect world.
The goal is to minimize the trusted computing base (TCB) so that it is, at least to some degree, verifiably correct. Then tack on features using isolated components.
In real life there are certain parts of the OS that have to work or the whole device stops working. Furthermore: the isolation of dynamic and less tested application code from these parts is generally a good idea, that's why monolithic OSes are so popular; they're simply less demanding.