I have no idea what the cost to society is. I don't even know if there is a cost. Allowing people to leave bad marriages sounds like a gain to society for me. Do you know the costs?
Unhappy marriage is a poor euphemism for neglect, abuse, beating.
That's true of many unhappy marriage, but not all (unless one stretches 'neglect' and 'abuse' beyond all meaning: if a husband puts on 200 pounds, he couldn't blame his wife for not being attracted to him).
There's a huge difference between abusive and unhappy.
But at what cost to society, those around them and perhaps even themselves? There are worse things than being in an unhappy marriage!