While I get Development is an abstraction of population; I don't like it. Its too abstract.
I mean, you could replace it almost with a different scale, like instead of going from 0 to 100 dev, you go from 0 to 100,000 population. Suddenly it means something to me.
Population numbers were fairly static in the middle ages so you could set it at the start of the game so that different areas have different human capital and are worth vastly different amounts. Right now I don't care if the county i gain in the war has a dev of 5 or of 40, it really doesnt matter all that much. But if i gain a county with 40,000 new peasants to work my lands and be levies in my armies, or only gain 5,000, that's a big difference.
It really doesnt need to be a complicated system.
But just switching the development number out with a population number wouldn't work (at least if you want any level of historical accuracy, which even if you don't, others on this forum will). As the economic output of an area wasn't just a function of population, but also of land quality, infrastructure, governance, technology, etc... For instance as populations grew in the middle ages, in some areas more and more marginal land area were worked meaning each added person gave less economic output (this is one reason the Black Death wasn't as devestating to Western European economies as the population decrease would suggest, as the more marginal farms were the ones that were abandoned). This phenomenon would be a nightmare to calculate in CK3 as counties are divided into vastly different sizes (not to mention the effects of terrain or cities). So when a population would start seeing diminishing returns on output would differ for each county.
So to get any level of accurate depiction of economic output with population you need to modify the population number's effect on the economy by something that represents the rest of what development currently stands for (for instance development could be split into a population and an abstracted infrastructure number). But getting two numbers to work together in a realistic way is way easier said than done, especially when one of those numbers is strictly bound by reality (this is why I think the Civ comparision is bad, as the population number in Civ are pretty much made up with little need to conform to the nitty gritty of reality). Also note, that by adding other mechanics that effect the output of each county's population, it also means that the total population of a realm is a less meaningful number. So this extra work is really being done just for flavour (which I'm fine with in principle, but it needs to be within reason).
So while I'd love a number that tells me the population of my realm (especially if broken down by culture and religion), I can understand why it's seen by the devs as too much work for too little gain.