Another argument against the great filter, and one I'm surprised it's not considered more often, is the relativity of the space and time. We assume, long story short, that the colonization explosion is rare because we can't see it, but what if all that we can observe in terms of space and time is a incredibly "microscopic" slice compared to all that there is/has been/will be out there? The way I see it it looks like we've concluded water must be commonplace on earth, but then we act puzzled when we can't find a lake down the street and assume water must be rarer than we thought.