The problem with C++ is that the various standards have encouraged changing idioms. Pre-standard, 98, 11, and 17 are significantly different when the new language features are fully leveraged. Legacy code creates inconvenient barriers when interoperating with modern stuff. Rust may be able to escape this problem.
Rust, unlike other languages I know of, has a well defined process to make breaking changes to the language. These changes are opt-in on a crate level so you don’t need to migrate an entire code base in an instant. The team also ships tools to auto fix code out there to make the migration easier.
The 2018 edition had a few small breaking changes, the 2021 edition will have none. Personally I hope the 2024 edition makes some breaking changes to make IDE development easier.
C++ is backwards compatible the same way and there was never any need to migrate your code. What happened was that c++11 brought in a major set of new features, changing the best practices of how to use the language, almost to the point of making it a new language. Mainly smart pointers, initializers, type inference, threading and lambdas, and removing tons of other small rocks in the shoe.
So that someone familiar with c++11 or newer opening up a legacy code would maybe not have a hard time understanding it but they would go "wtf is this verbose shit" and be tempted to rewrite it. Vice versa an old schooler would need to relearn these new features when opening a new code base.
Saying Rust isn't more difficult than possibly the most difficult thing is not a good argument in favor of Rust not being difficult, no?
People dread maintaining legacy C++ systems. People dread maintaining legacy systems. Rust will be no different.