I haven't studied the Benedict Option in-depth. But here are my three off-the-cuff reactions:
i) The future is unpredictable. I'm not an optimist or a pessimist. I don't know in advance how things will turn out. There are times and places in church history when Christians are forced into a very contracted existence. So there may be times and places where something like the Benedict Option may be the best we can settle for.
ii) But defeatism is a self-fulfilling prophecy. By assuming that the cultural dominance of Christian values is a lost cause, we marginalize ourselves. If you surrender, you're bound to lose. If you keep on fighting, you're likely to win some and lose some. There are many chinks in the secular armor. I don't presume that secularism is inevitable. But if you stop trying to stop it, your capitulation makes it unstoppable. The sure way to lose a tug of war is for one side to let go.
iii) Finally, it's naive. I'm reminded of Machen's analogous observation that:
It may well be questioned, however, whether this method of defense will really prove to be efficacious; for after the apologist has abandoned his outer defenses to the enemy and withdrawn into some inner citadel, he will probably discover that the enemy pursues him even there. Modern materialism, especially in the realm of psychology, is not content with occupying the lower quarters of the Christian city, but pushes its way into all the higher reaches of life; it is just as much opposed to the philosophical idealism of the liberal preacher as to the Biblical doctrines that the liberal preacher has abandoned in the interests of peace. Mere concessiveness, therefore, will never succeed in avoiding the intellectual conflict. In the intellectual battle of the present day there can be no "peace without victory"; one side or the other must win.
To build on Machen's point, secular progressives are utterly intolerant of Christianity. They are not content to deracinate religion from the public square. They won't tolerate the private practice of religious dissent. They won't tolerate politically incorrect beliefs. You will lose your job. Your children will be brainwashed in public school. They demand unconditional endorsement of the current liberal orthodoxies.