I don't agree fully. If some idea looks really good but implementations tend to be very problematic then the idea is likely presented incompletely or inaccurately, because it carries some hidden/non-apparent risk.
Some good-looking ideas almost always result in beneficial implementations, some good-looking ideas almost always result in bad implementations.
If all implementations of a "good" idea are bad then that's a strong indication that the "good" idea might have some significant flaws.
If the "good" idea has some bad implementations as well as some good implementations (like the swedish network example?) then perhaps you shouldn't dismiss the "good" idea so quickly
Sure, let's get to concrete things. What is a separate physical network worth, availability wise? Kind of hard to answer. It depends on the threat model. Even geography.
In this case though the two things are closely intertwined. The reason we all use the internet is because it is the most fit-for-purpose network for moving bits around between intranets. If there was a substantially more effective way to do it then it'd be cheaper or better and we'd all migrate to it over time. Countless businesses at all levels of the abstraction stack labour to make the internet cheaper and more convenient (CDNs are unbelievable, I say!).
So people choosing to create a new network are, with high confidence, going to end up with networks that are substantially worse at moving bits around cost effectively than the internet. The reality that they are inconvenient and expensive is built in once the deliberate choice is made to avoid the internet. It might be worth the cost, but the cost comes with the idea.
HSCN was said to be imperfect. It is inherent in the idea of building something like HSCN that sometimes the implementation is just bad in some aspects. actionfromafar's objection to that (idea independent from implementation) is invalid, because inherent in the idea of building something like HSCN rather than just using the internet is that implementations will suffer from relative imperfect. The fact there are relative imperfections is baked in to the idea.
Some good-looking ideas almost always result in beneficial implementations, some good-looking ideas almost always result in bad implementations.