Replace C++ code with Rust is happening at a glacial pace, basically involving full rewrites of the module replaced and significant work to refactor the surrounding code to support it.
(This is why Rust is focusing heavily on interop right now, if it is easier to integrate more projects can use it)
The problem is updating a decently sized program from one version of a compiler to another can take man months to do and that doesn't typically include intentional breaking changes.
A C++ change that requires updating 1% can be several hundreds of thousands of updates which is on the scale of beyond a man year depending on if you can regex cheat.
The fear the author is glossing over (I wouldn't say ignore they acknowledge there are reasons) is when faced with a man year to update people just don't.
Especially when the most interesting breaking changes don't tend to be synthetic (you can just make modules look like code you couldn't write before after all) but instead be subtle changes in behavior.
That means that you won't necessarily even know all the breakages which is a horrifying concept.