Re: I hear your argument
A good point. We may find no actual weapons, but may find more labs or facilities intended to develop them.
It is worth remembering that in the context of our future safety, a program to develop WMD is not much different than a stockpile of WMD ready to go.
If they find evidence of attempts to develop WMD, but no actual completed stockpiles, this would still be sufficient justification under the circumstances of Saddam's leadership and policies.
In fact, if they find the labs, but no weapons have actually been built yet, it is an even better justification, since it implies that we hit them at the one window of opportunity after they demonstrated intent to develop WMD but before they had actually done so.