Linux (like the BSDs) being C is an organizational phenomenon. There is no plausible technical reason for it not to be, or include parts in, C++. History is path-dependent; how we got here matters more than what is around today.
There is really no excuse for systemd being C. It is really a drag on its development.
UNIX and C are symbiotic, to get rid of C we need to get rid of both.
On the OSes that aren't just plain UNIX clones, C has a much less relevant role, and in some of them it is even being phased out, even if it might take a couple of decades yet.
At the time everyone was sure that Pascal was The Future of both OS and app development.
What happened instead was that everybody had their own Pascal dialect with the extensions it needed to be actually useful. Meanwhile, C had them all, already, and was about the same everywhere. Soon after, C++ came along, and there was again only one (although Microsoft delivered workable template support very late).
Lesson in Network Effect there. Also, shooting for usefulness out the gate.
Meanwhile, the Pascal crowd went off with Modula-2, Modula-3, Oberon, Apollo Pascal, UCSD Pascal, Object Pascal, Delphi, and countless others, none compelling. Apparently Delphi (still) has economic importance. The rest, not so much.
It used the Pascal calling convention, however Lisa and Mac OS were implemented in a mix of Pascal and Assembly.
Pascal UCSD also used a mix of interpreter and later AOT compiler.
Most mainframes don't use C as their original implementation language, and the oldest still being sold (Unisys ClearPath MCP) was designed about 10 years before C was born, in an Algol derived language, that uses instrics instead of Assembly.
If one broadens their horizons beyond UNIX, and looks into the history of computing there are plenty of interesting languages and OS architectures to dive into.
The Internet is full of digitalized version of papers and computer manuals from those days.
