“I’ve never seen a problem that wouldn’t be easier to solve with fewer people, or harder, and ultimately impossible, with more. Population is reaching its optimum and the world cannot hold an infinite number of people,” Sir David, who has two children, said.
Many problems easier to solve with more people...like "let's build this bridge". Quote out of context?

The native population of western countries is evening out/declining. So...we aren't going on as we have been. Improvements in technology increase the number of people the earth can support, and as countries become richer people stop having as many kids.

There's no doomsday scenario here.