Originally Posted by The Glyphstone
If the two sides are calling in outside reinforcements in that manner, the Orks are going to get flattened under the weight of the entire Swarm, and if it's limited to on-planet attrition, the Zerg can replace their casualties faster than the Orks can.
Can they? Where are they getting the biomass to do so? The biggest argument I have seen so far is that they will eat their dead/the Ork dead, but that just means that the Orks need to continuously advance. As long as the Orks don't allow the Zerg time to feed, they are locked into a 0 sum game. In such a scenario the Orks would win because they rapidly get stronger as they fight, and would have the bodies of their brethren to eat (they are made of the exact same substance as their food). The opposite is true of course, all the Zerg need to do is win battles to recover. So the wars outcome is based around whoever wins the majority of the early battles, and I would give that to the Orks.
Orks are by and large tougher than Zerg and they can recover from any wound (as long as they win the battle and can get themselves stitched back together). They have mechs which can be repaired via repainting them, and magic guns that don't need real ammo (the fluff says that many Ork bolters are nothing more than a box of metal with nails inside, which means you don't need complex manufacturing to reload them).
Invading groups almost always have the upper hand in the beginning due to concentration of force, which means that the Orks have the advantage in the pivotal early battles. If they win the early engagements they have removed the biomass that drives the Zerg, strengthened the psychic energies of their Waaagh!, and gained in physical size and strength.