Before somebody starts claiming about something making "sence", how about making a clear definition of what manpower actually is?
Because the way i see it, it is just a viechle allowing to create a situation where countries can reinforce fast due to pool of people readilly aviable, but then, the reinforcement speed dramatically drops due to the need to find man that are not readilly aviable, so manpower regain rate limits the reinforcement speed in lengthy war`s to realistic figures, effectivly forcing countries to finish wars sooner or later, since one side will gain an upper hand eventually due to different cassualty/mp regain ratios.
Adding the "max possible amount of people" the country can recruit is impossible without having a full set of pops, including age distribution, and impractical, due to all countries will go economically broke and starve/loose the ability to supply/pay armies way before they run out of man to arm.
As I see it, for this kind of thing gameplay trumps realism. Would having armies subtract from manpower improve gameplay? I think it would. You could also scrap force limits with this, and have manpower be a more "organic" limiter.
Armies already substract from manpower when building, and force limit is there to controll standing armies.
What you want to make, will just slow down the mid-late phases of big wars, due to manpower pool depleating faster, and the phase where manpower regain trumps the day comes in faster, and that is about it.