The point about "free concerts" such as those in Hyde Park is that someone had to pay for them, even if it wasn't the audience. The motives were partly altruistic but they were also a massive marketing exercise for the bands involved. See this link

The people involved had already made the money from more conventional music promotion, and chose to spend it in this way. Good for them. However they needed to have the money in the first place.

Conrad, the problem with your suggestion is not the bit about making it free for the audience, but the suggestion that it can be done for no cost.