> But zsh doesn't even use readline, which is bananas to me. Sure, provide an alternative but at least support readline.

Why? ZLE is what makes zsh compelling for users. It provides a high level of customizability into command line editing that is simply not possible with bash. It powers zsh's most touted completion feature as well as popular plugins such as zsh-syntax-highlighting[1] and zsh-autosuggestions[2]. Replacing it with readline would be a sure way to lose a majority of users in a blink of an eye.

[1]: https://github.com/zsh-users/zsh-syntax-highlighting

[2]: https://github.com/zsh-users/zsh-autosuggestions

.inputrc should be universal, and it is nearly. But not here.

I don't want to have customize each tool individually.

> .inputrc should be universal

Absolutely not. Using readline means completion and keybinding customization would be limited to what bash can do, or even worse as is typical with other readline based REPLs. What you're asking for is to gut a definitive feature of zsh only so that it can be bash. That provides zero value to zsh users.

You want independent projects to agree on something?

You do realize this is open source, right?

They don't have to agree on anything, just use readline. It's already used by, or optionally used by, tons of software.

Here's a non-exhaustive list that immediately come to mind:

  bash
  rc
  psql
  bc
  ftp
  GDB
  GPG
  Guile
  Lua
  NetworkManager
  Parted
  Python
  Ruby
  SQLite
  wpa_supplicant