I put it to you that a "shell" that does not natively run the executable programs you type is a non-starter as a shell.

I would be ready for the window if I had to type

    import subprocess; p = subprocess.run(args=["curl", "-s", "http://example.com/"], capture_output=True); fh = open("output", "wb"); fh.write(p.stdout); fh.close()

rather than

    curl -s http://example.com/ >output

There are certainly attempts to create a shell that's powered by Python and allows the Python language into it, but one could not "largely replace bash with the Python REPL". Only people who didn't want a shell in the first place would think that reasonable.