Not a regression. Most of the systems code you'll ever touch is still C, and knowing C makes Rust's unsafe blocks and FFI story way less mysterious. I learned C after Rust and it changed how I read Rust too: the ownership rules click differently once you've managed memory by hand and felt the bugs they prevent.
The first language is formative. It gets the programmer to think at a particular level of abstraction. If that level is wrong for the problem at hand, the programmer needs another language.
Why would a Rust programmer learn C? Isn't that basically a regression?
That also reflects in that many embedded systems offer C support and do not offer Rust support.
Write, say, a boot loader, for enlightement.