Not only will a Rust rewrite take 5-20x longer, you'll also be supporting 2 versions (C++ and Rust) for a long time, while doing the rewrite and migration, and that will eat up a lot of dev hours.
I've done incremental C++ migrations and they are not nearly as complex or stressful, especially when you leverage libtooling or Python bindings to write AST-rewriting scripts for those large refactorings.
If "Modern C++" really was a static target you could imagine performing the transition to "Modern C++" and you're done. If that existed it would compare to the Rust rewrite, maybe it would be 5-20x faster. But "Modern C++" isn't a static target, if we understand "Modern C++" this way (say, as C++ 17) obviously it's unsuitable for such safety of life software.
Instead the "Modern C++" transition is like a worse version of one of Zero's paradoxes. We never arrive at our intended destination and are always instead asked to perform another transition, the work never ends.