That's because it's not hard to demonstrate that this just isn't true. Ideas that aren't logically right get rightfully dismissed.
3.5 billion years is PLENTY of time for small changes to amount to a change of colossal magnitude. Simple example - continental drift rates average somewhere around 10% of the growth rate of your fingernails. In a billion years (do the math), that's more than fast enough for a continent to relocate halfway across the earth. You may think that's apples and oranges, but we have observed evolutionary rates that are more than fast enough to produce the kinds of change you're talking about.
But you've touched on one of the big problems many people have about understanding ToE: small brains (like mine, I admit) just don't handle really big numbers.
That's why Creationists get away with bogus analogies, like coming up with a huge number supposedly representing the odds against homo sapiens arising through natural selection, or the odds against a whirlwind in a junkyard assembling a 747 (which is so far removed from ToE--but I'm preaching to the choir).
It's just hard to grasp how just how long is a billion years. A half hour of Barney the Dinosaur can feel that long, and I nearly gave up hope that the interminable Clinton Age would ever end!