Microkernels lost "back in the day" because of how expensive syscalls were, and how many of them a microkernel requires to do basic things. That is mostly solved now, both by making syscalls faster, and also by eliminating them with things like queues in shared memory.

> you still need to design everything from the ground up

This just isn't true. The components in use now are already well designed, meaning they separate concerns well, and can be easily pulled apart. This is true of kernel code and userspace code. We just witnessed a filesystem enter and exit the linux kernel within the span of a year. No "ground up" redesign needed.