That points to another issue: sustainability. If the group is that small, and presumably aging, then how do you ensure you have enough in the pipeline to ensure you can support and enhance the Windows kernel for years to come?

Why do you presume that group is aging and not regularly rotating people? A dozen people sounds like a standard team size to me, which seems mostly indicative of the scope (not importance) of the work.

Also I believe Microsoft employs somewhere around 50.000 engineers, a good amount of whom work on and maintain low-level systems. I find it unlikely they'd have a shortage of sourcing talent for one of their core projects.

I imagine maintaining the kernel is very important. As that can be (and is) a major source of vulnerabilities. Also, it largely determines what hardware Windows can run on and what features it can have/support.

If the NT kernel is in maintenance mode that might be because they are planning on replacing it.

I recall Linus saying the same thing about waning numbers of linux kernel developers, so it seems like that is a problem affecting both kernels.