I personally like the fact that I can occasionally move into the prompt
and copy portions of it like any other chunk of text, so I think we
should generally (by default) refrain from making text intangible except
when *really* needed.
How about setting the below as default:
(setq minibuffer-prompt-properties '(read-only t cursor-intangible t face minibuffer-prompt))
I find having the above very risky in user config because it simply overrides whatever the default value of minibuffer-prompt-properties would be. Also using setq/defcustom is the best way; append/add-to-list/setcdr will not work very well because of the format of this variable's value. What I mean is that the list manipulation would have been much more convenient from the user end without risking complete override of the default value, had it been in an alist format like below:
(setq minibuffer-prompt-properties '((read-only t)
(cursor-intangible t)
(face minibuffer-prompt)))
The user then needs to add just one line to their config if they want to make the minibuffer prompts untouchable.
(add-hook 'minibuffer-setup-hook #'cursor-intangible-mode)
If you don't have that add-hook in your config, you will get the behavior you want, correct?
Summary:
(1) Proposal to set the default value of minibuffer-prompt-properties to '(read-only t cursor-intangible t face minibuffer-prompt). I believe this should go in emacs-25, not master.
(2) User can then put the following in their config to get untouchable minibuffer prompts: (add-hook 'minibuffer-setup-hook #'cursor-intangible-mode)