The Republicans were right: the United States does face an existential threat. They were just pointing in the wrong direction.
It seems like the Republicans have chosen to dispense with any nod to decorum and respond to Scalia's death with a game theoretic posture: they will not confirm any nomination the President sends to the Senate, no matter what. Okay, then, from a pure partisan game theory perspective, it seems like President Obama's move has to be this: accept that he can't get anyone confirmed and nominate someone who is unquestionably qualified and unquestionably liberal. Someone who doesn't demur to stare decisis and refuse to comment on cases that may come before the Court, but is willing to say, outright, that Citizens United should be overturned. In other words, someone to electrify the Democratic base, to turn the 2016 presidential and congressional campaigns into proxy Supreme Court campaigns.
This isn't how it's supposed to work. And, most likely, it's not how it will work this time, either. If past is prologue, Obama will once again hope that this time will be different, that Lucy will hold the ball and Charlie Brown will get to make that kick; he'll select a compromise candidate, maybe even a conservative jurist, maybe a "caretaker" of some sort. And, once again, he'll see that the response from Republicans is indistinguishable from the response if he hadn't made any attempt at compromise at all. He'll see all the downside and none of the upside. Again.
For Obama, what course of action is ethical, what is moral, what is political, what is best for the country's citizens, and what has the best chance to prevent a constitutional crisis? Each question leads to different answers. For the Republicans, they could realign all those questions into harmony simply by allowing normal business to resume, to stop obstructing for game theoretic advantage alone. But for years now they haven't, and they've baldly declared their intention to continue the blind obstruction. I do not think I'm overreacting when I say that tonight, for the first time in my life, I'm honestly afraid that the government of the United States might face an existential crisis—from within.