Obviously if the option was something like debug_print = false, then users should not expect their debug printing to keep working exactly as before. But Duration in particular should have Display IMHO. SystemTime won't have Display, but also its Debug impl is awful, so I don't think anyone will miss it.
I do care about all the issues I've listed, but most about small executable sizes. I don't want any Rust-generated strings in my executables, and I do not care what panics or debug logs look like in release mode. I do use build-std already for features like panic-immediate-abort.
I think eradication of parsing of debug impl's is important for the health of the Rust ecosystem (it's a hack, and if people continue to rely on it, it will ossify Rust's Debug and it won't be possible to change it in the future, and Rust will be left maintaining the worst reflection API). Finding it with miri is an interesting idea, but it just finds the offending code, but doesn't create a motivation to stop the bad practice.