It's too bad you've reached this point. You might consider allowing the players to roll back this event if they would be interested in continuing the campaign. In the future, consider more hints that the trap is coming and also provide ways that at least a few of those caught might survive if very lucky. I sympathize with the players. A TPK to an unknown trap just isn't fun.
One thing I've tried to do is always foreshadow the save-or-die situations. As Judge, that's something I always try to telegraph. For example, in Tower of the Stargazer (a module for Raggi's Lamentations of the Flame Princess), there's a dead thief who -- in the module -- is described as having fallen to his death. But the door has a save-or-die trap on it when a bronze snake/doorhandle bites a character. I changed the module to have the thief dead from some kind of snakebite. That way, when the characters walk up to the door and get bitten, it has more of an "oh, i get it now" response. It's less of a surprise and more of an explanation.
I think that crawlspace trap is adequately foreshadowed. I might feel bad about it if I didn't emphasize the puzzle-piece nature of the column in the center of the room. It's something that a Thief or Dwarf might pick up on innately. Something along the lines of... "Hey, these pieces are supporting weight..." or "Even savages aren't dumb enough to design a room that requires that much weight support in the center..."
In addition, the death of the PC is dependent on either a really bad Initiative roll or a bad Reflex save (or both). But that's hardly solace when they're re-rolling characters left-and-right. Some of the changes I've implemented in TA/TG are a response to this sort of reaction to the capricious nature of death in DCC. I think it's entirely appropriate to the genre and style of play. I mean, I prefer it as both a player and Judge. But many roleplayers are no longer accustomed to their characters dying in this way.
That's why, as a Judge, I foreshadow the heck out of save-or-die situations. And I'm also more likely to require multiple saves for save-or-die monster abilities -- with the character suffering irreparable damage on the first failed save and death on the second. And it's also why I made some of the changes I did in TA/TG, as well as giving players and Judges more choices in regards to character death.
I'll often warn players too when they're faced with a critical life-or-death roll. In the case of that trap, I'd warn them that characters who roll below a 20 on Initiative are likely to die. And I'd recommend they spend some Luck. This sort of advice has helped players in my group. Half of them are 4e players too. So there ya go.
We've had more near deaths than deaths since we started TA/TG playtesting. But we had a few deaths in DCC too. Nothing like multiple PC deaths per player, though. Which is probably more due to my style of presentation than anything to do with the game. I mean, 30 years ago I would've totally sprung a deadly trap on players without thinking twice. Now, I'm more reluctant to do so. Sometimes I feel like my job as Judge is more about emphasizing the threat
of death than killing characters left and right. It's just how things have drifted over the last 3 decades.
I'm actually quite surprised that the TA/TG characters have survived this long, although one of them has recently been attacked by a feral vampire and it's likely she's on the downward slope of mortality.
And back to the "what do I do now?" situation of DCC and this group.
That's a hard one because a lot of times I've found groups operate on their impression of a game. For example, my local group felt that DCC was really stingy with Luck points. When in reality, I didn't have access to the Judge's chapter giving advice on how PC's should recover Luck or get it back. I informed them that the issue wasn't with the game. It was with me. And I rectified my approach to Luck, Alignment and all that sort of thing to tweak it more towards the group's expectations. I'm doing what the Judge's chapter says. Just maybe a little more often and obvious
than the advice would think necessary.
Maybe that approach would work?
I mean, what we're really fighting in this situation is the group's first impression of the game. I wouldn't be squeamish about using the Recovering the Body rules on p. 93 in this situation. It's not my favorite rule. But if you haven't made much use of it, you could be like "whoops! hey guys, there's this rule I totally didn't understand and this guy on the forum totally pointed it out to me and it's my bad. Roll to recover and take 4 XP because I goofed up."
Something like that could maybe turn it around.