Parry, head of the development, concepts and doctrine centre at the Ministry of Defence, is charged with identifying the greatest challenges that will frame national security policy in the future
To be fair, its his job to think up the worst case scenarios. As reported by the Times, nothing in what he was saying seemed at all impossible, indeed it all seems fairly reasonable. Its not his job to predict the most likely future, its his job to try to prepare for the worst possible one.

Indeed, in some respects the situation is worse than he seems to be suggesting. The article posits a large gender imbalance in thrid world countries as a destablising factor, but the age imbalance that we already know will unquestionably happen is likely to be just as destabalising without the need to imagine any one family-one child policies.

Average age in the UK today: 39.3, 17.5% under 15
Average age in Italy today, 42.2, 13.8% under 15

Average age in Iran today: 24.8, 26% under 15

And so on. Go...figure...