There's no economic reason the nominal GDP of any country or the world in general has to continuously increase. The important metric is per capita production. As long as people get continuously more productive through innovation, standards of living will continue to increase.
At the national level, vying for long term economic power in the world, a higher and younger population is going to be a huge advantage very soon and countries should be trying to get as many immigrants in their borders as they can. But instead they are...going a different direction.