Even if it's not what is used by default, I've experienced and heard about enough other people experiencing pains with the prompt_toolkit-based input breaking people's workflows, that I think we should seriously consider bringing that code back in.

I think it was a mistake to remove it altogether, and change the default at the same time.

• recent discussion on the Sage mailing list about this
• Add multiline coding in ipython 5 with --simple-prompt #9816 has several users reporting breakage
• there are several users brought up prompt toolkit having regressions compared to readline in prompt_toolkit ignores .inputrc #9388
• and similarly search backwards by default #10075 contains sentiment from a user saying this: "Multiline editing is a very nice feature. As a longtime vim and bash user, though, the switch to IPython 5.x and prompt_toolkit has been jarring."
• Use vi mode for prompt_toolkit layout #10085 lists a limitation and inconsistency of prompt_toolkit which will not be addressed upstream.
• Graceful forced exit? #9944 is another issue that came up with the prompt-toolkit transition, which used to work in old readline based code, (though I understand Thomas recently proposed a fix for this in Exit terminal interface gracefully on signals #10185, there is discussion there about the limitations of that, too).
• %magic% doesn't work in Emacs 25.1-2 ipython (run-python) buffer on Windows 7. #10211 is an emacs user affected a change that as far as I can tell is also attributable to the transition to prompt toolkit.
• ipython 5 tab completion not compatible with jpype #10315 is another unexpected breakage in IPython 5, because prompt_toolkit uses a separate thread for completions (whereas completions had to be synchronous in readline, so jpype completions worked fine in IPython <= 4).
• Multi-line input only processes the first line #9948 - user unable to paste multi-line using tmux.
• IPython terminal prompt missing after upgrading from 2.3.1 to 5.1.0 #10071 - randomly missing prompt after upgrade to IPython 5 inside docker (because terminal size was reported as 0x0)