"Demographic decline" is when the population of an area shrinks because more people are dying off than being born. From an economic standpoint, this is bad for a few reasons (regardless of the underlying economic system)—

• You could lose economies of scale with things like public infrastructure. If you have millions of people contributing to the upkeep of, say, a metro line via the fruits of their labour (directly or through taxation), then that's all fine, but if your population suddenly shrinks to half of what it was, the cost of upkeeping the metro line hasn't halved, but the amount of total wealth in the area has halved, as now there are only half as many people contributing to the communal wealth.
• The labour pool has shrunk. This is important because some things have a fixed amount of labour required and do not scale well. For example, a train always needs exactly one conductor regardless of how long it is, but now you only have half the number of conductors available. Again, it doesn't matter whether we're looking at a communist or a capitalist system, it's easy to see how this is bad in both systems.
• People tend to get poorer because the vast majority of economic activity (and therefore wealth) is generated through people interacting with each other. This also results in a reduction of government budgets because revenue is generally tied to group wealth and economic activity. This happens at a disproportionate rate compared to the actual percentage of population loss, meaning the amount of revenue per capita goes down as well.

Some countries are already experiencing demographic decline. It's bad for everyone, but it's much worse for rich people than for poor people (in absolute terms), at least in capitalist countries, because rich people have much more direct exposure to the macroeconomic forces that result from it. Additionally, a lot of rich people's wealth is tied to growth. Infinite exponential growth is impossible; it's not a sustainable model for wealth. But they have a lot to gain from at least postponing the pyramid's collapse until after they die.

Maybe not a million dollars, but something like a $50,000 cash bonus per child successfully raised to adulthood and has graduated school without being expelled would motivate a good number of parents. The $50,000 is a paltry amount compared to the economic activity that a person generates throughout their lives so it's a great deal for the Government, even when combined with things like childcare subsidies, etc.

The second condition would encourage parents to deal with delinquent behaviour and keep their kids in school. If the kid gets expelled, you lose the $50K.