First of all, there is no such language as C/C++.

C++ although tainted by copy-paste compatibility with C89, does offer the language features for security conscious developers to make use of, and C++17 is quite a pleasure to use.

C on the other hand could be nuked for what I care, it has been clear since 1979 that security and C would never go together.

Now regarding your assertions.

z/OS was initially written in a mix of Assembly and PL/I. C and C++ came later into the picture as the system got a POSIX personality.

The way to write libraries exposed to all languages available on z/OS is via the z/OS Language Environment.

Chapter 9.12 of z/OS Basics, https://www.redbooks.ibm.com/redbooks/pdfs/sg246366.pdf

.NET has hardly any pure C code, rather C++.

Even so, the C# and VB.NET compilers have been bootstrapped thanks to Rosylin and now C++ is gone from the compiler side. F# was bootstrapped from the early days.

Since Roslyn came into production, the .NET team started planning moving parts of the runtime from C++ to C#.

"So, in my view, the primary trend is moving our existing C++ codebase to C#. It makes us so much more efficient and enables a broader set of .NET developers to reason about the base platform more easily and also contribute."

https://www.infoq.com/articles/virtual-panel-dotnet-future

Also the first version of .NET's GC was actually prototyped in Common Lisp,

https://blogs.msdn.microsoft.com/patrick_dussud/2006/11/21/h...

Unity introduced HPC# at GDC 2018 and they are now in the process of migrating C++ engine code into HPC#.

Both the requirements of HPC# as C# subset for performance critical code, as the experience with Singularity and Midori drove the design of C# 7.x features regarding low level GC free data structure management.

"Evolving Unity"

https://www.youtube.com/watch?v=aFFLEiDr3T0&list=PLX2vGYjWbI...

"Safe Systems Programming in C# and .NET"

https://www.infoq.com/presentations/csharp-systems-programmi...

"Safe Systems Software and the Future of Computing"

https://www.youtube.com/watch?v=CuD7SCqHB7k

Dalvik was written in C++ and is dead since Android 5.0.

ART was written in a mix of Java and C++ between Android 5 and 6 as AOT compiler at installation time.

It was rebooted for Android 7, where it is now a mix of an interpreter written in highly optimized Assembly, with JIT/AOT compilers written in a mix of Java and C++, making use of PGO data.

On Android Things, userspace device drivers are written in Java.

https://developer.android.com/things/sdk/drivers/

Sun back in the day toyed with the idea of having Java on the Solaris kernel,

https://www.researchgate.net/publication/220938922_Writing_S...

And on SunSPOT devices, http://www.sunspotdev.org/

Fiji VM and PTC Perc Ultra can be compiled AOT to native code and run Java bare-metal.

https://www.ptc.com/en/products/developer-tools/perc

http://fiji-systems.com/

Oracle is also in the process of following JikesRVM example, and with the help of Graal, bootstrap Java thus reducing the dependency on C++, via the Project Metropolis.

https://www.youtube.com/watch?v=OMk5KoUIOy4

Graal, which incidently has better

LLVM is written in C++. Yes it does expose a C API, but it also has bindings for other languages.

At WWDC 2017, Apple announced that launchd and the dock were rewritten in Swift. I expect other OS components to be announced at this years' WWDC.

Windows kernel was written in C. Since Windows 8, C++ is officially supported on the kernel and given the company's stance on usefulness of C, they have been migrating the code to compile as C++.

"We do not plan to support ISO C features that are not part of either C90 or ISO C++"

https://herbsutter.com/2012/05/03/reader-qa-what-about-vc-an...

Now Visual C++ has been updated up to C11 library compatibility, as per ISO C++17 compliance requirement, that's all.

"We have converted most of the CRT sources to compile as C++, enabling us to replace many ugly C idioms with simpler and more advanced C++ constructs"

https://blogs.msdn.microsoft.com/vcblog/2014/06/10/the-great...

https://www.reddit.com/r/cpp/comments/4oruo1/windows_10_code...

Fuchsia's TCP/IP stack, WLAN services, disk management, package manager, update service are written in Go.

https://groups.google.com/forum/#!msg/golang-dev/2xuYHcP0Fdc...

https://fuchsia.googlesource.com/garnet/+/master/go/src

Genode OS, ARM Mbed OS and Arduino Wiring are written in C++.

As I mentioned, change requires replacing generations one person at a time.

First lets turn C into the COBOL of systems programming, then we worry about C++ afterwards.