What is the problem with enforcing this rule. It was designed specifically to make an official's job easier and safer, and makes perfect common sense. You gently remind the Head Coach of the rule, and it's reasons, during the pre-game discussion, politely remind coaches who might forget, or get carried away a little, a time or two ("Coach, I need the sideline, thanks), and if necessary, a sideline warning is most often sufficient.

If your initial requests are ignored, and a side line warning doesn't serve as a reminder, a 5 yard penalty is an absolute "wake up" call that any coach should be enbarrassed needing. If that doesn't work, the individual is likely being stubborn or deliberately acting as a smart ***. Then it's entirely up to you, how much you want to put up with that.

Lower levels should command better reaction, and require less patience.

I don't see a problem with enforcing the rule. I do find that it's better to be proactive and remind coaches to move back "Help me out, coach, I need that space..." rather than throw a flag right away and create an antagonistic situation where one wouldn't otherwise exist.

They have a job to do, I have a job to do. Most times we can co-exist quite fine on the sideline without me having to "be the bad guy."