I do not consider the presence of .idlec in $HOME on Windows to be a problem in itself.  There are numerous applications, unix-derived and otherwise, that do the same with config files.  Very few, if any, have the issue of sharing between multiple simultaneous versions. Installing a new versions deletes the old.  Many do not have the issue of users needing to edit the files.  These would be better candidates for making the switch.

Under current circumstances, I reject the proposal.  However, since I hope circumstances change, I am closing this as 'postponed' rather than 'rejected'.  What I hope we can do, to make me *want* to re-open this.

* Switch from almost-dependable sockets to always-dependable pipes and delete the backup single-process (-n) mode (see IDLE Help).

* Switch from one user-code execution process to one for each editor window (including the shell).

* Let any recent (and installed) python version be chosen for each execution process.

* Improve configuration handling to where the need for user editing is mostly gone.  Example: ability to add a custom theme from a file without the user copy and pasting.

Each of these has benefits in itself, and the net effect would be that most everyone would only need to run the latest IDLE they have and not need to touch the .cfg files.  At this point, we would want to change the format and put them in a different (and less accessible) place anyway.