Well, how may people write code in assembly now?

I can tell you that I used to do a lot of assembly. I even wrote an X86 compiler for fun, played with the Z80 to write Gameboy games, and did PIC Microcontroller stuff for a while when working on an engineering team.

I don't think I could write anything meaningful for any new processor, really. I could write enough to get a driver working or something, I'm sure ... but memory management and multi-threading?

Truth told, we already have that gap. No one is writing much of anything substantial without tools that handle the low end stuff. Most new languages don't even allow the developer to manage memory, and even when you do it's mostly 'virtual memory' as presented to the program by the Operating System, because the 'real' memory is being managed a layer below most running programs.

We keep abstracting things away, for different reasons.

Most developers have never written a simple 'hello world' in assembly, and even computer engineering students, who's entire purpose is to understand that level of computers probably haven't written anything that really uses the full feature set of a modern processor.