A reply to /u/the_claus

Yes, rulers and their advisors were very conscious of demographics. This does not mean they had good information, nor did they choose the best policy.

As an example, I wrote here about the dilemma facing Philip IV where decades-long dependence on Castile to shoulder the burden of empire both financially and demographically started to show it cannot last forever.

Thus, he started to look to Aragon and Valencia, which economies was seen to be rising, and to the entire domains of the King of Spain in Iberia and the Mediterranean. He envisioned a new system whereby each province should contribute their fair share. After much deliberation and calculation, he presented the scheme of the Union of Arms in 1624, apportioning various states and provinces to raise men and pay for their maintenance such that the sum would be 140,000 men. Basically, he wanted a national army composed of contributions from each state and province.

Of course, each state at this point had their own court, privileges, and laws regarding taxation, contributions, and how to control its army. Naturally, the Cortes of the states weren't accepting of this proposal. Olivares' counter-argument is that the numbers he came up with were based on estimates of wealth of each state, and in particular Portugal and Catalonia were two wealthy states that had not contributed nearly their fair share thus far. His estimates proved to be incorrect, as there were no means of estimating wealth nor population, and with political pressures ended up back-firing big time against Philip IV.