If anything, proponents of this idea think too small. If our "advanced" technology were back engineered from crashed saucers, then why don't we have faster than light star drives? Why don't we have aircraft capable or instant right angle turns? Instead of progressing naturally, why hasn't our medical and engineering knowledge advanced exponentially?

You can never get a "straight" answer from proponents when you ask questions like this.

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"The facts gentlemen, and nothing but the facts, for careful eyes are narrowly watching." Isaac Asimov