Living standards are not declining world wide? You have a source for that claim? AFAIK, there has been a little decline in past couple years due to recession. Otherwise has been improving steadily for decades.
I depends on where you are and what the expectations and definitions for "increased standard of living." Increased sanitation and food? Certainly, however that means a proportional increase in population.
One may argue that it's not a law of nature that it happen, however human history has demonstrated that populations expand to consume available resources. The tendency would be to put more people at risk and head towards the miserable lives they once endured. There are solutions of course but they are more or less unacceptable to most cultures living in poverty. Having a large family is often considered culturally desirable whether it's for social standing or having kin managing a farm and providing for family members as they age. Education would help but it runs counter to culture then the latter wins except in the very long term. An example I'll cite is female circumcision. Many erroneously assume that it's male dominance attempting to enforce fidelity, but that incorrect. It is men in many situations who are the progressive force against it while the women themselves steal their own daughters away to have this done as a right of passage. If something so obviously hurtful is supported, how is anyone going to convince people that they need to have fewer children who will then turn their parents out to let some imagined benign government to care for them?
Perhaps as bad or potentially worse is the idea that everyone should be Westerners, consuming things in excess that they don't need and only want because they are told they do. While consumerism may encourage technical progress I believe it is the ultimate Ponzi scheme based on the illusion of unlimited resources. At this time we are dealing with shortages of needed materials because they are indeed being used up. The idea that the environment can be modified to give allow the modern appetite for material goods to spread to one and all isn't realistic and indeed I believe our way of life is as deleterious as any adherence to religious tenets. We must stop being our things.