I agree with the OP. He's not asking for a population mechanic, just a simple cosmetic correction to increase the immersion.
I'd argue that is even worse than people asking for an actual population system, as for just an immersion value we then need to track more info on every single province in the world and simulate that for when it increases and decreases only for it to have no actual game mechanics tied to it doing anything so it just becomes an unnecessary hit onto performance.
Development is already the abstraction we have over this and related infrastructure to implement it to a level we deem necessary in Crusader Kings relevant to how prominent a thing it is in the design of the game, I'm sure there are some ways it could be improved no doubt but I fundamentally don't think a flavour number or even a full population system have much to benefit a game like Crusader Kings where the focus is squarely on the characters
