So you save a few milliseconds if you don't initialize some filesystems, good but makes very little difference in the end.

Not having a hard disk? Doesn't require a microkernel at all. Booting over LAN has been practical for a long time.

If I glance the closest linux machine I have, it has a 6MB kernel with 10MB initrd. To put that on a server with gigabytes of ram? Trimming an unused networking stack here is not the use case where a microkernel will help performance.