A good safety protocol incorporates redundancies. The "warning" you're arguing against is secondary to first and foremost carefully checking for approaching cyclists. Both the parent and I were clear about this. (I'll add I'm horrified by everything else the parent wrote.)

> Given space they might stop, which is pretty discourteous by the person opening the door

Precisely the opposite is true: thoughtfully opening one's door slowly as a backup mechanism in an effort to prevent someone else's injury is very courteous. As a cyclist myself, I'd far prefer to be inconvenienced by an unscheduled stop than I would crash into a car door.

> with insufficient time they might well swerve into a traffic to avoid the collision, which could be far worse.

The alternative in your scenario is the driver impatiently swinging their door open. In this case the cyclist might well serve into traffic, too. Your argument makes no sense to me.