Based on the theoretical possibility that other universes could exist, that is a valid point. If it could be proven that this is the only universe, then the creationist argument is not only credible but virtually irrefutable.
First of all: I reject your argument that proof that this is the only universe would make that particular pro-Creationist argument more "credible" - let alone "virtually irrefutable." You have provided absolutely no reasons for jumping to this conclusion (namely: "This is the ONLY universe / no others exist" => Intelligent Creator).
You and I have ALREADY agreed that WAP does NOT address the "likelihood" of "fine-tuning." But in this latest argument of yours, you are now sneaking in the concept of "likelihood" through the "back door." Specifically: You are essentially positing a "likelihood" of 1 (our ONLY universe) divided by ZERO (the now-alleged total no. of all possible universes). As you know, dividing by ZERO is meaningless.
An even only cursory semantic examination of your statement reveals that you are rapidly approaching a point at which nonsensical suppositions and self-contradictory propositions must be entertained, if this particular pro-Creationist argument ("fine-tuned universe" => Intelligent Creator) is to be maintained.
Proving that this is the "only universe" = disproving the existence of "other" universes (even hypothetical ones?!). This might be compared with proving that "1" is "1," and not some other number. The very word "universe" contains within itself the presumption that it includes EVERYTHING. In my previous comment (#19), I even addressed this issue by including the proviso:
*I am not claiming that actual "multiverses" necessarily exist. WAP certainly doesn't address the existence of any other universes or demand that they must exist. One could instead speak of "hypothetical" [emphasis added] universes, if you wish.
Regards,