I think the hybrid approach, high-level Rust + low-level C is actually preferable. Rust really shines at the high-level things and you do what C is good at on the drivers.
You can translate C line by line into identical rust at no cost except that you have to write "unsafe" once per fn. With the exception of macros, but rust's replacements there are just better.
Keeping everything in one language seems like a worthwhile reason not to use C to me.
Proprietary software for this is also available with superior UI and features, and I expect people serious about high-reliability embedded systems already use it.
Rust Nightly does have limited support for printing stack usage by function with -Z emit-stack-sizes, but I could not find anything for analysis of worst case stack usage in whole programs.
At the $15 end of the market, developers have a good chance of wanting to import code from other sources. For example, depending on the application, they might need an RTOS to provide multi-threading, FAT32 for an SD card, or a network stack with SSL implementation.
Often, the developer will want a 'lightweight' library, i.e. one intended for microcontrollers, and often this will be written in C.