Customer asks why I won't just "vacuum it out" while I'm on the roof

So I'm up on a two story in Springfield doing a standard sweep on an old brick flue and the homeowner yells up asking why I can't just run a shop vac from the firebox and be done in 10. I explained that a real sweep needs rods and a proper brush to knock the glazed creosote off the walls, and a vac alone just stirs up the soot into the air. He tells me his old guy used to do it in 15 minutes flat and charge him $60, which tells me everything about how that flue got so bad in the first place. I pulled out a chunk the size of my fist and held it over the edge so he could see it, and that finally shut him up. What do you guys say to folks who think sweeping is basically just cleaning with a hose attachment?