Like poster upthread, I figure you're ok because you could cut expenses if you really need to.
Taking your question on its own terms, though, the issue seems to be whether your Social Security is enough to compensate for the missing 400k in establishing a safe 72k income. 'm not an expert, so I'll just offer my reasoning and let you decide. Someone smarter will probably offer a better model and I'll learn something.
Social Security's income adjusts to inflation, so the amounts you have are relatively safe there. Risks are primarily whether the govt cuts benefit rates. The SS fund is supposedly on track to cut benefits about 30%. You could put SS at 70% for each of you in cFireSim and see what you get. It won't be 100% success, as you already learned. So you can't get a "yes it's safe" unless you can accept some risk.
In the end you'll be balancing a few percent chance of needing to cut expenses vs the gain in life enjoyment of quitting work now. What that chance is depends on the assumptions you use. The life enjoyment is personal decision. A judgment call eventually.
IMHO you should also ponder the chance of dying in the next 15 years - you'll have enough money during that period regardless, but the time spent off the job will only come if you seize the opportunity. I bet that the chance one of you will die in 15 years is larger than the chance you run out of money in 40. Add up the probabilities from the Social Security's actuarial table. I think you'll see over 20% chance one or the other goes in the shorter time. Seize the day!
https://www.ssa.gov/oact/STATS/table4c6.htmlI guess the judgment issue is separate from whether you are completely safe. cFireSim is under 100% already so you're not guaranteed 72k if you retire. You wouldn't even be guaranteed if it was 100%, you'd just have good odds. So you'll have to decide for yourself which is more important: the last few percent of financial near-certainty, or several more years of work.