The issue is not what we know now, but what was known then.

A lot of people thought that Saddam had active WMD programs and large stockpiles. This includes almost every intelligence agency on the planet, most of Saddam's neighboring nations and their leaders, and a lot of people in Iraq, including Saddam's own generals.

Given a lack of human intelligence resources in Iraq, misinformation from some informers and spies*, a wealth of suspicious activity seen from satellites and other sources, Saddam's known and proven history of deception and secret development of WMD, and the U.N's less-than-stellar record of discovering such hidden programs, the assumption was not unreasonable at the time.

You can give someone grief for thinking there are fish in a lake when it turns out there aren't any, but if his estimation was based on observing the lake only from the surface, and yours is based on what is found after the lake has been drained, that doesn't automatically make you more perceptive or the other guy an idiot.

What we know now about Iraq's weapons programs would have been utterly impossible to find out without having invaded the country. There is no way on Earth even the most thorough inspection process permitted by Saddam would have come up with the information we have now.

And there are still a lot of holes left in your usual "slam dunk." The deliberate destruction of many files and sites before we invaded. Lines of trucks to Syria, a nation currently working on WMD itself. Large numbers of weapons scientists on the payroll years after the new Common Wisdom says Iraq abandoned all programs. Assassination attempts on these scientists even after the war.

By the way, given the conclusion that Saddam would have become dangerous when sanctions were lifted, what would your course of action have been? Keep them on forever? (How many people a month dying as a result of sanctions?) If not, what then?

*The source of many of the specific "locations" mentioned in intelligence reports