The main obstacle to making Rust a "C++ but better" language is arguably the half-baked design of its support for pinned data. (Because "pinned", i.e. location-sensitive objects that can't simply be memcpy'd elsewhere are ubiquitous in idiomatic C/C++.) But there are several language proposals to mitigate this in the future, though for most of these a full transition will require an edition change in order not to break backward-compat.