Excuse my poor choice of words. "Irreducible" complexity has been sort of hijacked by the Intelligence Design folks; I should have just more clearly focused on the probability involved in a change from simple chemical compounds to complex biological systems. Mea culpa.
As for the rest, it is back to math. Pascal, whose name we all remember from our early school years sitting in arithmetic class, was the father of Pascal's Wager, considered "groundbreaking because it charted new territory in probability theory," according to Wikipedia's account. In a nut shell, "Pascal argues that a rational person should live as though God exists and seek to believe in God. If God does actually exist, such a person will have only a finite loss (some pleasures, luxury, etc.), whereas they stand to receive infinite gains (as represented by eternity in
Heaven) and avoid infinite losses (eternity in
Hell)." (Again, Wiki).
When you look at the three tiers of probability that you must navigate (compounding as they add up) to get to a universe, simple life, then complex life forms absent a creative force (as thinly laid out in my previous post), and the numbers become so great that the probability of considering a creative force at work has at least equally remote chances, then Pascal's Wager (the probability-driven argument that "a rational person should live as though God exists and seek to believe in God" (extrapolated: creationism) has a new heft to it.
In other words, Pascal's famous wager tells us that--all things being equal--if you believe in God (allow me to stretch that to "creationism"), you have nothing to loose, but if you do not believe in God ("evolution"), you have everything to lose. That "all things being equal" part (my injection) comes into focus better when you compare the improbability of a creative force and the three-tiered improbability of universe, simple life, then complex biological systems absent a creative force.
Stated differently, if it is improbable that a creative force led us to advanced life forms, and there are three separate levels of compounded improbability to get to those life forms absent such a force, a betting person (or an actuary) would go with the former. And...Pascal's Wager tells us that you face gaining everything if you do so (even if the odds were not so stacked).
So...the single argument that I put forward is simple math (probability, more precisely). When you look at the actual numbers tossed about in the current writings on "fine tuning," the 10 to the nth, where n is so many zeros it crosses your eyes, it is a very hard argument to dismiss without the employment of logical fallacies.