Valid, but here's something additional that ties into that complaint to think about: a "new viewer" in this case can be (and will often be) somebody who has played tf2 before and understands the map. The reason this allows them to engage in understanding AD more than 5cp is because there is less difference between how one would play in a pub and how one would play in a comp game.
They can look at 5cp and understand how to win a round but they don't understand why teams can't push chokes, why teams have to give up ground with uber disadvantage, why a med/demo pick is better than a roamer pick, what a roamer even is, what offclassing is and why it's situational, why you sac people in onto last, why people use cookie-cutter lineup, I could go on and on.
If you look at AD you see a team set up with attacking classes, and a team set up with some defensive classes. They push the points and the team defends the points. They might not understand the finer strategies but they understand and can follow the flow of the game, and can empathise with the situation a lot better.
I agree with you in the sense that a straight A->B (->C if needed) map would be awesome but I don't think changing gravelpit is the answer, I think it needs a different map.