The case for "100% Safe C++" is that you might be able to annotate that old C++98 code in ways that don't otherwise alter its semantics, but still ensure safety. That would be a one-time cost that might be well-worth paying if the cost is low enough - Where "cost" depends on developer experience as opposed to mere volume of annotations. A "viral" compiler feature that auto-surfaces all the places that will need annotation for a given level of safety has the potential to be quite easy to learn and use effectively. It's not clear why the C++ folks are rejecting that approach, seemingly out-of-hand.

I have > 10 million lines of C++ that is not annotated. There are many projects much larger than mine. If you cannot automatically annotate the code there is no point in trying as you can't do it manually. If you can automate it why not just build that into the compiler and skip the syntax?