> If this project really did require re-inventing C++ in C, it must be justified by being fairly large. In which case, a low-end ARM (Cortex M based) microcontroller would have been entirely suitable. All ARMs have multiple C++ toolchains which support them.
ARM is not entirely suitable for any application just because you have complex (for some definition of complex) software.
No, but the software impact of choosing a particular piece of hardware should absolutely be part of the system design process. If it is not, then you are part of a project with bad leadership, or very, very old-fashioned architects.
It's not just ARM - there's plenty of other CPU architectures with modern Clang, GCC, or other toolchains with C++ support. Given that this is a large code base (justifying this amount of work), this cannot be a "tiny" microcontroller, as in 8 bit, and needing to run on microamps. I cannot believe that there were not C++-capable microcontrollers available which would have done the job, without breaking the bank.
