The rational argument for what is being proposed is that companies with (total) hundreds of millions of lines of C++ code can migrate it incrementally and (hopefully) mostly mechanically as the language evolves ... which should be much cheaper and less risky than porting code to a new language.

There are probably also strong non-rational components, e.g. C++ people would like to keep working on C++, leveraging their existing skills and status, rather than jump to another community.