pfSense

1. Button: 'copy'
2. Popup: which fields to change (interface);
3. Save = copied with altered values.

You're assuming everyone uses it the same way you use it, which isn't the case. Removing functionality to prevent foot-shooting isn't a good idea.

I'm assuming people want to work efficient.

What is wrong with copying a field into a field to make sure people don't get RSI and don't waste their time on filling fields manually that a computer can do for you? Because that was my main proposal.

I'm used to a change mode versus a display mode, which is why I suggest read only or hide. But that was the detail, not the main suggestion. I can understand your point for this detail, so scrap that detail. It leaves the main suggestion.

Let's assume I need to do work on 600 VLANs. A (computer) system that automatically copies field A to field B avoids a lot of useless clicking.

I mean it constructively, btw, not to whine or something.

But the details you mention are not solved by this suggestion. The interface is already filled/selected when you create or copy a rule. To lock that would reduce functionality (the ability to edit and move a rule).

Locking the field would take away the ability to edit a rule and move it to a new interface in one step. That would make moving a rule a much longer process because people would have to duplicate, change the interface, and then remove the old rule.

So in attempting to solve one problem you've made a much more common use several steps longer and more inefficient.