Existing languages that have evolved by adding features onto a decades old base have a large technical debt. Sure, it is possible to write relatively safe code

Existing languages that have evolved by adding features onto a decades old base have a large technical debt. Sure, it is possible to write relatively safe code in C++ using modern features like std::unique_ptr, but you have to consciously use those features and any time you actually want to make use of that base (say, by using an older library), you lose all those modern benefits, at least in a part of your code. You either have to intertwine old code with new or write bindings, just as you would with Rust. No software can keep total backwards compatibility for decades and be truly modern and consistent, because old mistakes and now unwanted features have to stay. To even attempt having the best design possible you have to either break backwards compatibility in new versions (like Python) or make an entirely new language at some point (like Rust).