Try doing an Austria-Hungaria run now ._.
"Planning ahead" there is imposssible. you have 0 manpower and you have to go to war before you can raise your manpower. but in the meantime you're fighting (read, defending) with 0 reserves, and the fighting drains more then the daily trickle supplies.
How am i to survive the second world war when i cant even reinforce troops? Getting shelled by CAS isn't helping either. I have to relly on myallies to defend my airspace, something they don't reliably do
Sounds okay to me. Sucks for gameplay purposes but your enemies face the same problem. Basically now you will sometimes be forced to think about switching to higher recruitment instead of "oh boy 0 manpower, lets scrape the barrel for millions of reserves instantly". You will also have to pay attention to war support since that increases mobilization speed.
Its another layer of gameplay, if you play a country with lower base manpower you can either join "the good guys", take advantage of defensive war and get manpower more quickly, or think before you mass click on recruitment button and start training 50 divisions like you used to do before WtT. It will be interesting to see how people use strategies based on this in MP, do you use all of your manpower on 50 divisions to overcome the enemy or do you save it for prolongued fighting? System is not fully explored yet.