Not defending yourself means that the other side wins a default judgment.

“That said, other countries will frequently enforce foreign judgements. In this case, the specific performance they're asking for would likely be unenforceable in the US (and useless regardless) -- but damages might well be enforceable.” writes Greg Maxwell on the Hackernews thread already mentioned above.

The biggest issues are that a default judgment may still financially impact the defendants, and a default judgment may establish a precedent that open source developers do owe their users duties, even when the license explicitly disclaims it like the MIT License in Bitcoin Core does.

It seems to me that fighting the lawsuit is globally less of a headache than ignoring it. As expensive it will be and as stupid as the lawsuit is.