For it to be enough something like at least 70% of humanity has to be concision and willing to do something about it. That's not the case, the vast majority of people do not care, and so the efforts of a couple will not be enough. What will happen when the few that do something about it die, and not enough younger people replace them?

In a sense it reminds me of investing. Put all your eggs into one basket, win 3 times, but if you lose the 4th time, it's too late and you lose everything. Eventually it will be too late and everything will be lost.