Actually, I don't think your argument proves what you want to prove.
I don't know of any theist who would say that God COULD not have created a system in which species become more and more complex. That's THEISTIC evolution, in which God designed and guides the process.
The irreducible complexity argument of Behe is used to challenge ATHEISTIC (Darwinian) evolution, which requires a mechanism involving UNDESIGNED, UNGUIDED, changes over time. Irreducible complexity argues that these gradual and random processes are insufficient to account for the complexity of living systems.