Looks like the only thing left is the rest runner, and it's probably not happening anytime soon since it's a ton of syscalls and Rust doesn't really add any advantages there imo.
The impression that I got is that while there is nothing more to port in the shell itself, the performance and concurrency goals haven't been fully realized yet. The new Rust code needs a bit more tuning before it can replace the old shell.
fish was a great shell when I tried it, but it's unfortunate that so much is written in bash. The most interesting shell by far was xonsh though a python powered shell
Fish's main attraction was never its scripting language - it was its UI. You can run bash scripts from fish (using bash of course), while still enjoying its bit more modern UI. Bash was never a challenge to fish. Nushell, on the other hand, is a different case.