There are two risks: death of the wage earner, and job loss. The impact of death can be mitigated with insurance, job loss can be mitigated with savings.

But it also probably means living a less robust lifestyle to keep expenses reasonable for a one-income household. The trap many DINKs fall into (not accusing you, just riffing on my observations) is living a lifestyle that's on the edge of affordability for two earners, and thus feeling like they HAVE to have two earners indefinitely.