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.

I agree with opening the door a crack as the final check, after you think no cyclist is coming. It seemed the parent post was advocating opening a crack as a first line check. It was that I was opposing.

How is that any different than cars and turn signals?

The obligation is on the party obstructing the traffic flow to not be a dick about it (which includes broadcasting their intentions to some extent) and the secondary obligation is on the moving traffic to not do ludicrous speed so that the former party can accurately predict whether or not they can make their move without being a dick about it.

If I were cycling past a car and the door began to open, I would (and do) respond on the assumption that that door is going to open all the way. You don't have to be moving fast for that to be problematic. Moreover, there are plenty of places you have continuous parked cars. If you slowed to a genuinely safe speed you may as well be walking.

Dutch cyclist here :-) you slow down a little, look over your shoulder to see whether there is space and move over. If there is a car at slow speed behind you they see this "look" and typically slow down or make some space as well in anticipation. But if there's a bus at high speed or an asshole when you look over your shoulder, you stop before hitting the door.

All of this is easier in Dutch cities with bike lanes and because we cycle much slower than the typical "rider" in San Francisco or London I've seen. Mostly because people commute on old slow bicycles in normal clothes instead of light weight geared bikes and full on sports outfits with helmets.