I mean, it shouldn't! After some time, you'd be using a newer version. You want to be using an old rustc to compile a project created later in time. There are very very few languages that do not add new features over time. Even Go adds new standard library functionality or toolchain features in new releases.

The Python 2 to 3 situation is very different, that is about new versions not compiling old code. We go to great lengths to make sure that that works well in Rust.