Yes, there have been some failures, mostly from the corrosion of the air inlets on the front struts and yes, they aren't cheap to repair. However, your assertion that the "closed" system on the Jeep would have a catastrophic failure it there was a leak isn't accurate nor is the assumption that the systems on the Cayenne/Touareg/Q7 are "open" systems in the traditional sense of the term. Closed refers to the way air is channeled to and from the shocks. In the slightly older systems used on the pre-2011 Porsche/VW/Audi pressurized air to the shocks is managed through a manifold system fed from a pressure tank. The "pump" is used to pressurize the tank, not the shocks. On a traditional "open" system atmospheric air is used to pressurize the shocks so it can't react to fast, minute changes. The major difference is the way they dump waste air. The older systems vent to atmosphere which means repeated actuation results in the potential to exhaust the pressure supply and the system has to wait for the tank to be filled by the pump. The Jeep, OTOH, "pumps" the air back to the tank from the shocks. It has to pump the air because the tank must always be at a higher pressure than the shocks. On any system a failure in the system can cause a failure if it occurs within the air bladder. If it occurs in the lines or the tank there are check valves that will close off and prevent a suspension collapse. Air leaking out isn't a problem either, they all draw atmospheric air to recharge the tank if the pressure falls below a certain point. You can purge the tank and the system will refill it through a one way valve.
You can argue all day which system is better and whether it's reliable but that's academic IMHO.
However, if you are someone who will purchase a GC and run it well past warranty then perhaps the air suspension is not for you. Of course no one knows if you can retrofit the metal suspension but based on what I see visually I think that would be a very easy modification many years and many thousand miles down the road. And of course like any other technology it's something more that can break.
Sounds like if you don't want air suspension then an OL isn't for you though I found at least 1 at a local dealer that was early production and does not have QL so if you look hard you may be able to find one.