Well, the horns certainly block some of the top 1/3 or so of the radiator. I think the grill probably does as much harm to the airflow in that area, especially at highway speeds. A significant portion of the airflow comes from under the bumper. I think you could argue that the air flow at the top of the radiator is already messed up because of the grill. Maybe putting the horns doesn't make it a lot worse than it already is. You'd need to do some actual testing with equipment to verify this.

I live in Texas, with temperatures around 100F for much of the summer, and no winter to speak of, but I have no trouble with either the A/C or overheating so there's not a real need for doing this.

My Mercedes had the horns in front of the radiator also, as did a BMW I drove for a while, so it's not as though Volvo has done something unique.

You want to talk about poor design? Let's discuss the window switches under the cup holder that get condensation moisture from cold drinks in them (or stuff spilled in them), the A/C evaporator failure rate, the ABS modules that go bad, the ventilation recirculate door link that breaks once a year on manual systems, and probably a few others I can't recall at the moment.

I don't see it as a problem unless the fan or thermo switch breaks.

The engineers didn't miss it, that's why a fan is there. Until I see some real evidence that removing the horns would make any difference other than making the fan run less, I wouldn't consider moving them.