> are people writing an entirely different shell interface.  They're rare.

Or just using a different shell interface.  Or even different defaults for the standard shell...

And they are not rare.  Almost nobody in reality use CPython shell - that's definitely true for scientific applications of Python, data scientists, etc.

> As your IPython example showed, their job is harder than _just_ replacing PS1/PS2.

Yes, custom shells have much more stuff (In/Out arrays, etc), but changing PS1/PS2 - require very tiny modifications for doctest.  And such changes - almost all that usually prevent using standard doctest for testing examples, written for applications, which assume "a different shell interface" (which, I remind, may be just a plain CPython shell with customized ps1/ps2).