That's like saying that there's a spectrum from "planet teeming with life" to "lifeless planet" and then sticking "planet does not exist" onto the lifeless planet end of the spectrum.
You can talk about a spectrum between "planet large enough for life to exist", "planet which is a little smaller which makes it a little harder for life to exist" all the way down to "no planet with no life". You can do that because in this spectrum, the extent to which the planet exists and the chance of life are connected. In the God example, the spectrum is "degree to which things can be explained without God". If you have evolution, there is one less thing that you need God to explain, and you get one step closer to not needing God to explain anything. And with nothing to need God for you can then reject the existence of God.
You could do a planet-size-spectrum like that; but reversing the analogy would be - I don't know, some sort of spectrum ranging from "God doesn't exist" to "God exists"? That seems a pretty binary set of points to me - how can the state of "God 50% exists" make any sort of sense?
And with nothing to need God for you can then reject the existence of God.
Similarly, if nothing that you ever see requires the existence of Jim, then you can reject the existence of Jim, right?
Another month, another rationality quotes thread. The rules are: