I'm all for replacing readline. It's an ancient, very closely linked to the everyday experience of using the command line, and not very good. I want my REPLs to have things like syntax highlighting and decent tab completion UI, and readline has positioned itself to have a monopoly on the ability to implement these things but failed to deliver.

But this isn't addressing the real problem, which is that readline gets conjoined with the programs that use it. So if I want to write my own line editor, I have to significantly modify every program I want to use it in, which is enough of an obstacle that I won't do it (and neither will others). Nevermind more difficult architectural problems, like making editing local when on high-latency connections. Still, having a BSD-licensed line editor to start from may be helpful for whoever takes on that project some day.