The trouble with that is that you can't change it in case e.g. some temporary visitor to your house snaps a picture of the key.

If you as an individual has visitors who come to your house and snap pictures of codes in your devices... I wouldn't be more expectant of Zigbee or any other standards, rather I would be more keen on the visitors I allow into my house.

That shouldn't do them any good. The device should require it to be in pairing mode for the key to do anything.

But then what do you need the key for? In theory somebody else could try to pair with it while you have it in pairing mode, but they would have to be on your property for that and could potentially see the key anyway. So you go slap them with a trout or call the police or something, and then go back to setting up your future garbage.

So that that initial pairing is more secure.

Zigbee etc. tries to get around the whole thing by requiring that the pairing process is done from a 5cm or something distance.

Imho that is good enough but I don't know how far you could extend it with directional antennas etc.

That anyone can reclaim a device they hold in their hand is a feature.

I'm not a fan. If I get a new hub, the last thing I want to do is unscrew every light switch to find the picture on it to scan and if I buy a house with pre-existing hardware I don't want to rely on the last owner having taken pictures associated with every switch and left it for me.

OOB pairing could have rolling keys, but that's more expensive. In general the problem is difficult, every solution has a nasty caveat, be it taking over during installation or what your described.