That turned out to be an interesting read. Let me adress some points and expand a little on my point of view.
Factorio is my favourite game (sorry KSP). With about 1.5k hours sunk into it I've never even touched multiplayer button. And it's not about traumatic expieriences. I don't think I have any. I simply get the most enjoyment when playing alone. Yes, I would like to bring a person or two into my modded Factorio Space Exploration campaign, but it would simply be a pain to find the time to actually play together. Either someone looses a lot of progress or we don't progress at all. Couch coop, hotseat, in-person board games - it's all fun. Once my only option is someone random through internet connection, I'd rather not.
I don't see many possibilities for grieffing in KSP. What's the worst that can happen? Someone landing a duck-shaped blockade in the middle of my runway, or attaching to my space station and deorbiting it. It's a bit too soon to speculate how syncing player's timelines would be done, but those problems may not even exist. Think about Souls-like multiplayer when you can opt-in or out of others worlds at will. If my station is safe, because others only ever see it's "shadow", then there is no place for intentionall carnage. And again if there is an option to "enable PvP" and battle that one friend, that's cool.
Honestly I expeced myself with "3.14% chance of happening" to be at the end of the spectrum. I didn't want to be too sugestive with the poll itself (I've seen some bias-inducing polls at this forum), but rather to get a general overview.
One more thing: I'm happy that Nate focuses dev team so much on new player expierience and multiplayer. Will it affect me? Not directly. Does it mean that I'll get to play KSP2 later than I would if multi wasn't a thing? Yes. But my altruistic side says: if it means more players get to enjoy KSP, it's well worth additional time.
There is also other side of this coin (that may have impact on my expierience with the game): modders gonna mod. Multiplayer codebase can get them to do things that couldn't be possible without it. Just imagine doing air launch mission and then desyncing time to actually both orbit one vessel and land another without resorting to quicksave loading and sacraficing one in "true timeline".