And how would you do that exactly? There are no accurate population census for most of the earth at that time. At best you'd have completely inaccurate estimates.
And why would You want accurate numbers? Or even if they'd be "accurate", showing numbers of every single person in province, than what's the problem? As I understand, pop system from Vicky would be totally unacceptable for most players, but simply number of people in province, their religion and culture (which we can directly influence- very easily) could be present (as well as taxation- they should have some reason to revolt finally, right?). Number of people would diminish over time if province is occupied by enemy.
Estimated numbers of people in certain areas are available, while indeed inaccurate- it's still much better than completely ignoring it.