That reply makes me so frustrated that I'm afraid to reply myself. If those are the only differences, then there's no point in holding Worldcon. Surely one of you can come up with a better answer than that?
The reasons why Worldcons cost more than other SF conventions are not relevant to this discussion. You have to think it terms of the person who has never attended a Worldcon, but has attended his/her local cons. Make the case for why you should spend three to five times more money for something that on the surface looks like the same thing.