I think with microkernel architecture this could have been very different. Even touching the kernel would keep outdated only a tiny part of the system with a relatively small attack surface, the rest could still get updates.
Your microkernel would either perform way worse (eg worse power consumption), or have pieces so tightly coupled that you'll wonder if it was worth it.
I think what makes it hard is the dizzying variety of CPUs that makes it all hard to keep up with. I have heard from insiders that as far as manufacturers are concerned, a CPU stays "current" for about 6 months.