Eh, I think a better analogy is trying to douse an inferno with an inferno. Tyranids gain biomass from this war, as well as newer and better ways to kill orks. Orks aren't advancing technologically to kill tyranids, but they would be getting a tougher hide, stronger bones, overall more resilient, as well as increasing in physical strength. Eventually one of them is going to lose, it's less a war of numbers and more a war of evolution. How far can both species go before they reach a biological plateau where they can just no longer advance. And we honestly can't say which would be the first to reach the plateau.