How much of that existing code, really, is ever going to be updated to the standard of even C++11?

I am working on a device, where the initial code was written in 1995-1997 on a pre cpp98 compiler, the code has recently been updated to cpp11, I am sure we will move to cpp14/17 soon. So this is slowly but steadily moving, even if very conservatively.

On the other hand, Hell would freeze over before you could convince the business to throw the cpp codebase and replace it by anything else, without necessarily a lot of things to show to the customer. And as much as we like to criticize business decisions, here it is probably the right call.

However, if you are startup starting today, it would probably be a good idea to consider other options, that could be used as an advantage against incumbents.

Plenty, AUTOSAR now requires C++14 compliance for example.

Lots of code might not be updated, thing is, with C++20 I still get to compile my pre-C++98 code.

Even taking into account the breaking changes between language revisions, they are quite minor versus writing into another language.

Plus there are still lots of stuff missing, like a proper GUI library that can even compete with MFC, OWL, let alone something like UWP or Qt.

There are 10 million lines of libreoffice code that has been updated to use these features

More than nothing, less than everything.

And that will be still be able to interop with the "everything else" that doesn't get updated.

(Perhaps not seemlessly, but certainly better than a Rust <-> C++ interop.)

A lot of it won’t.

But the interop story between modern cpp and older cpp is much better than the interop story between rust and older cpp.