I'm optimistic about binaries providing completions, I think we'll see more of that as tools adopt frameworks like https://github.com/clap-rs/clap or https://pypi.org/project/argcomplete/ which can provide completions for your favorite shell.

But I do think many important/useful binaries will never provide completions, or provide sub-par completion that people will want to improve, so we can't ever count on completions from binaries.

Recently I've been thinking some about how the LSP (https://microsoft.github.io/language-server-protocol/) might be useful for a CLI. A readline implementation could use something like a LSP for syntax coloring and completion.

Imagine having a consistent editing in your favorite shell, repl, and editor.