Well, I'd certainly be more willing to pull out my credit card for a $5 charge than I am for a $20 charge, which means it'd still be more successful with me than it is on the shelves at Walmart. And since I've never purchased a comedy DVD from anyone but threw down $5 without hesitation for Louis CK's special, it's infinitely more successful than the physical model in my case already.
How often would I do that? No idea. Depends how many comedians I really wanted to support, how often they put stuff out, and how my finances were looking at the time. But all three of those factors equally impact their chances at my money in "the real, physical world" (which is an utterly stupid phrase in this context, btw - he made $250,000 very real dollars in 4 days), and I'm still more likely to spend $5 than $20.
The only variable here is exposure. Can Louis (or other people who want to try the same) reach enough people using non-corporate-media methods to make up the difference between a $5 sale and a $20 sale, or can't they? Obviously a brand new talent isn't going to have the success with it that Louis CK does, but a brand new talent, with the rarest exception, isn't getting shelf space at Walmart either.
This strikes me as a lot of doom and gloom over nothing. He paid for production costs with ticket sales for the live show. He made back his money on putting the thing online more or less instantly. Will it work for everyone? Of course not, nothing works for everyone. But I bet it'll work for him again the next time he does it.