Cat 6 rider
Look at the old Slingshots. Their designers figured out that the downtube is always in expansion (edit: correct word- tension, thanks to BentChainring), and doesn't require a lot of strength, so they replaced it with a wire. I wouldn't worry about a dent there. Most bikes are way overbuilt in that area. If you have a catastrophic failure and end up on your face I didn't just say this. If you have a catastrophic failure and end up on your rump and winning $10 grand on America's Funniest Home Videos I get half.