Actually, the Jiangs should have done something - anything - to prepare for an attack, because it was very obvious that an attack was coming. It was, as op said, inevitable. But YZY was too classist and thought the Wens of all people wouldn't dare.
When the Wens arrived (again, as op already said) what they actually wanted was the place. If the Jiangs had surrendered they could've bought enough time to at least save themselves and their disciples, they could've loved to fight another day. It was YZY who escalated the situation into an all out fight by attacking WLJ when she found out the Wens wanted to make Lotus Pier into a supervisory office. She did that because of her pride. In fact, when YZY attacked WLJ she made a point of saying she was the superior from a famous clan and WLJ was inferior - it's YZY's classist world view that wouldn't permit her to let herself become the subordinate of a person she saw as her inferior even if keeping her pride meant sacrificing the entire Jiang sect and even her own life.
WWX had been willing to sacrifice his hand. WWX had wanted to do something other than sitting on his ass and waiting for the Wens to come since a long time ago. WWX was the one who knew from the beginning that the Wens would dare to do anything. But WWX was the son of a servant, so YZY refused to listen.
Quite frankly, the death of the Jiangs was the consequence of long term problems that had been festering in that family for a long time: toxic family dynamics, classism and bad leadership.
YZY and JFM had a disastrous marriage - largely because the Yu clan pressured the young JFM to marry YZY even after he'd already refused and had made clear that their personalities weren't compatible until JFM was forced to give in - and that meant that the two leaders of Lotus Pier were in conflict all the time. If they couldn't even put their own internal problems aside to deal with an external threat, how could they stand a chance when one did appear?
WWX did what he could to save himself and the other hostages with him. People who say WWX shouldn't have done anything are ignoring the context in which his actions took place: he didn't just rebel out of nowhere. WC had dragged all the hostages to a cave where he planned to sacrifice one of them in order to attract the XuanWu of Slaughter and capture it. Now, I think everyone is quite clear on WC and his followers skill levels, right? There's absolutely no way WC would have been able to kill the XuanWu, he would've panicked and fled, as did in canon, and abandoned the hostages to die. Everything would still have happened more or less the se way, with WWX and LWJ distracting the XuanWu for the others to flee, becoming trapped in the cave and eventually killing the XuanWu, being rescued, etc. WC would still have gone back and been jealous that WWX and LWJ had killed the XuanWu and the Wens would still want to make an example out of someone because the hostages had fled, and they did want Lotus Pier because of its location, so it's the ideal next target. That WWX was someone WC was envious of is only a bonus for WC personally. The Wens would still have attacked eventually even if WWX didn't exist. The only difference here is that if WWX had done nothing at the cave, the outcome would have been the same but the body count on their side would've been higher.
The only reason JC and YZY kept blaming WWX was because both of them had a victim complex and insisted on blaming others for everything that didn't go their own way, all the while refusing to take any responsibility for anything, even their own selves. This, more than anything, is why JC is an effective foil to WWX and LWJ. JC acts like a entitled child all throughout the novel, and a violent entitled child at that. WWX and LWJ are conscientious adults who take responsibility for their own actions and are willing to be responsible for the safety of others as well, and they are fully capable of shouldering that responsibility. We see that contrast in the way JC refuses to aid his own people unless it can bring him glory or somebody dies first versus how LWJ goes where the chaos is and helps anyone with even minor things, how he's willing to be responsible for LSZ after WWX's death, we see it in how WWX is willing to be responsible for the Wen Remnants at great personal cost, on how he cares for the Lan juniors since the moment he's resurrected and the other juniors later on, etc.
In any case, my point is that the fall of Lotus Pier and the death of the Jiangs was largely due to a management and leadership failure, that was the result of preexisting problems that had been left to fester for a long time and made them unable to handle an external threat. It absolutely wasn't WWX's fault.
