Well yes, it kind of is the idea to eliminate all the stockpiling hassles.
I agree that the up-front vs. upkeep cost thing was/is a really nice touch and I added it intentionally, so I bleed a little to give it up. However, there is no way to maintain it entirely without it being very gameable - i.e. recruit tiny unit, by all the paraphernalia you want, recruit all the other men.
There can certainly be a way to balance this while maintaining the maintenance payment idea. The important point is that raising a siege-worthy army should be possible. It's possible now because they can be bought a long time in advance for basically free, so a production of 1 or 2 engines per day can be sufficient.
Maybe an idea would be to allow easy drafting of extra siege engines?
