Fish is a shell, warp is a terminal emulator. So even if someone uses fish they still need a terminal emulator; e.g. iTerm2 or Terminal.app. Am I missing something?
Warp goes a bit beyond a normal terminal emulator and blurs the line a bit in order to support multiple shells without the user having to configure it in their shell by editing .bashrc or whatever. Eg Clickable position for text entry, which most other text boxes have supported since the 80's. I'm sure there's a readline setting somewhere out there but Warp overrides whatever settings are needed in order to get that to work out of the box.
Yep! fish and iTerm2 make a wonderful pair on MacOS, it's my go-to combo. My larger point is that you can add nice features to your terminal without adding cloud logins or shady proprietary clients.