The single point attachment at the seat stays or brake bridge does not have to be particularly strong. It's only there for for/aft stability.

The most stable racks have some triangulation when viewed from the rear. A rack with all parallel vertical members will be much more prone to swaying. Larger diameter tubes help, but that's a "band-aid" for lateral stiffness. If you look at the last vertical member in that photo of the old Blackburn rack, note that it is narrower than the two other members. They're the ones carrying the pannier load, but that last one is what keeps it from swaying.

Does that work? And last?

What they should be doing is powder coating them at the factory, especially since, IIRC, these aren't inexpensive racks. IIRC, they don't offer them in plain metal finish either, just their crappy black paint. It's a real shame, because they are great racks, very durable, tough, and for those of us with heel strike issues (big feet or converting old MTB's to tourers), being able to set the panniers further back is very helpful. Luckily the bike I have it on, the beat up look kind of fits, but if this was on one of my clean and shiny builds it'd really be bugging me. Yeah, I'm like that.

I've gone over 45MPH downhill through mountainous areas, fully loaded. My converted MTB, racks, and load were rock solid. There's nothing like the feeling of hauling ass on a fully loaded touring rig and have it feel right because it's nice and steady. One of my favorite things about touring. On quiet country roads and keeping to my own lane, I don't see anything "insane" at all about it.