There is where the judgement come in.
Your red-line may not be the same as the diver in distress.
If all you want to discuss is benign, no-stress, no-real-problem situations, fine.
But ANY of those situations can turn to crap in a few seconds.
One of my seminal dives was as an open water diver... It was a big boat dive in Jamaica and everyone was playing follow the leader with the DM. I was hanging out at the back of the group as we went into a swim-through. There was a group of divers who'd just finished their OWD training the day before, at the resort, and some of them were near the back as the DM entered the swim-through.
By the time the last 5 divers (out of 25ish?) were entering, it was completely silted out. I'd already decided that I was going to swim over, but I was watching a woman from the "just certified" group. I could see that her anxiety was climbing, and then her husband swam over to her as she sat on her knees near the entrance. They had the following conversation via hand signals:
Him: (Angrily) Swim through there.
Her: No.
Him: (More angrily) You swim through there and I'll follow you.
Her: (Angry) No, you swim though there.
At this point the husband swam through into the silt. I'm 100% certain that the woman thought she was alone on the other side. I looked at her stream of bubbles and I could tell she was about to panic and probably go for the surface.
Not knowing what to do, I just winged it. I swam up and asked her if she was OK, and she said she was, though I could tell she wasn't. Then I asked if she would like to swim over the swim-through with me to which I received a very emphatic affirmative. I remember thinking that I should hold her hand, but I didn't know how she'd take that. We did our swim over and everyone lived happily ever after.
I surfaced from that dive resolved to get more training. I knew I didn't know how to handle her if she'd have bolted, and I resolved then and there to take a rescue course.
I think most instructors/pros have a pretty well-honed ability to identify when someone's going to make the jump from problem to panic, but that's because they see it all the time. That certainly isn't an exclusive club, anyone can recognize it, it's just a matter of knowing when and how to respond. It's probably better to move close and try to help.
If I see someone get entangled, I'm going to move in to help them if that's going to solve the problem more efficiently, whether that's a dsmb or fishing line... Same goes for a tank slip. It's easier to have someone else do it.
There's a reason, I think, that there's no underwater signal for "I need assistance" and that's because when we are below the surface there's a tacit agreement that I will come to your assistance if you have a problem that I can identify.