I was responding to the particular one factor argument claiming hat defection was the rational strategy, which wasn't correct even with that factor in isolation.

For your point, as your probability of dying increases, so too does your need for cooperation to avoid it. The closer you are to risk of dying, the more likely you will need help to avoid it, so the more you would want to encourage cooperation. Again, the argument that it is rational to defect does not hold from this factor alone either.

But it isn't that it's necessarily rational to cooperate either - it's just that the trade off from defection versus cooperation is an empirical matter of all the factors in the situation, and arguments from based on one factor alone aren't decisive, even when they are correct.

As for an experiment, it wouldn't show what it is rational to do, only what people in fact do. If you had lived a life of cooperation, encouraging others to cooperate, and denouncing those who don't, the consistency bias would make it less likely that you would change that behavior despite any mistakenly perceived benefit.

There would be a billion and one factors involved, not the least of which would be the particulars of the experiment chosen. Maybe you found in the lab, in your experiment, that age correlated with defection. It's quite a leap to generalize that to a propensity to defect in real life.