I’ve had enough double-redundant links fail - and I mean pretty thoroughly double-redundant, different provider, different media, different physical path - that I’m quite surprised something like the air traffic control system only has two links.

Contractually dual-path, or actually dual-path? My understanding is that there's enough infrastructure horse-trading going on behind the scenes that it's very difficulty to be certain that two circuits between points A and B don't share the same infrastructure somewhere in between.

First big oops of this form that I remember:

"In December 1986, the ARPANET had 7 dedicated trunk lines between NY and Boston, except that they all went through the same conduit -- which was accidentally cut by a backhoe. "

https://www.csl.sri.com/~neumann/insiderisks06.html

One thing that needs to be reviewed is if they are dual links or redundant links.

In any system where both sides of redundancy always carry traffic then loss of one link can cause congestion failure if any link fails. This is a very common means of failure in electrical networks that requires load shedding. Well, you can't load shed air traffic.

A more complex system that I like, but comes with it's own set of constraints and implementation issues is a system where both lines carry all the traffic at all times. This way the default state of the system is always working and your first failure isn't invisibly critical.

But this is very hard as we see in TCP when things get out of order and high latency creeps in. You have to manage a lot more state at the data level.