Skepticism is healthy and encourages discussion. I am not observing a "collective refusal" as you put it, but instead a strong skepticism from the tech community (a community who has a good basis to interpret this story with).
"Do you all also doubt that they were behind the other hacks mentioned by the FBI (e.g. on South Korean banks and media outlets)? Is it 'incredibly dangerous' to accuse them of that as well?"
The discussion is around a specific cybersecurity attack, not North Korea's capabilities, so that's a bit of a straw man argument.
"I understand the post-Iraq mentality of demanding evidence before going to war with a country, but that's not what's going on here."
No rational person is making a case for war over this, but the fact that you mention going to war over this demonstrates why it is incredibly dangerous to make these assumptions about North Korea without looking at all the available evidence.
"Also, given how mouthy North Korea is, nobody finds it telling that other than a single denial on December 7th, they've been quiet?"
Quite the opposite. I think it would fit the profile if North Korea was being mouthy at this point in time. Their silence would suggest this is a delicate situation for them politically and they don't know how to respond to it.