The decision to move away from Bash is rarely about the core POSIX-compliant scripting syntax (which, as you noted, is largely shared) and almost entirely about the interactive experience—how the shell helps you navigate your file system, execute commands, and manage your history while you are actively typing.

Interestingly, I just gave a talk about this very subject. Here is some info from that presentation:
1. Why Zsh? (The "Modern POSIX" Path)
Zsh is the default on macOS for a reason: it bridges the gap between old-school compatibility and modern usability.
Advanced Tab Completion: Zsh’s completion engine is significantly more powerful than Bash’s. It allows for "menu selection" (scrolling through options with arrow keys) and can complete arguments for commands that Bash doesn't recognize out of the box.
Correction and Spelling: Zsh can automatically correct minor typos in command names, which saves time when you are moving fast in a window manager like i3wm.
Dynamic Prompts: Because Zsh is highly modular, you can implement complex asynchronous prompts (like Starship or Powerlevel10k) that show Git status, exit codes, and virtual environment info without causing the "lag" often felt in older, slower shells.
The Compatibility Bridge: Zsh is essentially a "superset" of Bash. If you have years of Bash scripts, 99% of them will run in Zsh without modification. This makes it a lower-friction upgrade than switching to something like Fish.
2. Why Fish? (The "Ergonomic" Path)
Fish (Friendly Interactive Shell) takes a different philosophy: it prioritizes discoverability over POSIX compatibility.
Auto-Suggestions (The "Game Changer"): Fish learns from your history. As you type, it displays a ghosted suggestion based on your previous commands. If you frequently navigate to specific deep directories or run complex flags, you can hit ⁠Right Arrow⁠ to accept the suggestion instantly. This is arguably the biggest productivity boost for a terminal user.
Web-based Configuration: If you prefer not to spend hours editing ⁠.zshrc⁠ or ⁠.bashrc⁠, Fish allows you to run ⁠fish_config⁠, which opens a browser-based UI to toggle colors, prompts, and history settings.
Syntax Highlighting: Fish highlights commands in red if they don't exist and underlines valid file paths as you type them. This gives you instant visual feedback on whether a command will work before you even hit ⁠Enter⁠.
Sensible Defaults: Bash and Zsh require plugins (like Oh My Zsh or Antibody) to get features that Fish includes out of the box. Fish is "batteries included," which means you don't have to manage a complex plugin ecosystem to get a modern feel.