EU like Manpower is a terrible mechanic and should even be removed from EU itself, but at the same time Victoria 2's system is indeed not very user-friendly and moreover, does not reflect the way armies worked historically. It is important to have soldiers tied to POPs in some way, but the system where people are supposed to join the army willingly based on the level of salaries and then form "Soldier POPs", does not have any basis in reality.
Most governments had a fairly good level of control on the size of their armed forces during the period, which was determined based on the state finances and willingness to spend money on the army, this was definitely not something left to market forces to decide. And soldiers from all parts of the country could be easily shifted around and grouped together to form regiments, at no point the fact that a "Soldier POP" in a specific area is depleted should be any issue to raise the remaining soldiers as a unit. Moreover, the overwhelming majority of the soldiers were actually drafted from the general population for a relatively short time, and did NOT constitute permanent "Soldier POPs" with distinct ideologies and preferencies, something Victoria 2 completely fails to represent.