@anp personally I'm interested in no_std / no-alloc use cases, but in such use cases fallible collections are provided by crates like heapless.
These crates do impl traits like Extend, but the problem is with a fixed-sized container Extend is fallible, and when it fails it panics, hence my interest in TryExtend.
If we're implicitly going off feedback on Rust in the Linux kernel, panic-free code is important as well, so it'd be nice to have some core traits for fallible buffers which are also panic-free.