However, if they were transit-free, they would, by definition, never have more than one peer for any single-homed prefix.
And that sounds like a single point of failure to me. Let's look at it another way by considering the path to any prefix. If there is only one single path available, and a single event, such as the depeering by one ASN, can lead to that path being broken, then you have a network whose connectivity is not terribly robust. If a network bites the bullet, and either openly buys transit, or works out some partnership peering plus transit deal to hide the fact that they have transit, then there is the possibility of having two paths for every prefix. If they then take the trouble to analyze the paths and adjust things to make sure that the multiple paths to a single prefix don't share fate, then they stand a good chance of having a robust network. The thinking, and the work involved, are a lot like what you need to do in order to ensure physical separacy of fibre paths. It's the same fundamental problem but perhaps more dynamic since circuits tend to get groomed less often than paths change. --Michael Dillon