It's a good idea. It would give small developers a chance to get on their feet financially. But the author assumes most of the revenues come from big-selling apps. I don't think we actually know how long a tail the app sales distribution has or what effect it would have on Apple's revenues.
And, unfortunately, it wouldn't have any effect on, e.g., Amazon being unable to sell books on Apple products because Amazon's margins are too low to allow them to eat the 30% and jacking up their prices 43% to cover the Apple tax would make them uncompetitive with Apple's book store. Apple's plan, maybe, but damned inconvenient for users.
And, unfortunately, it wouldn't have any effect on, e.g., Amazon being unable to sell books on Apple products because Amazon's margins are too low to allow them to eat the 30% and jacking up their prices 43% to cover the Apple tax would make them uncompetitive with Apple's book store. Apple's plan, maybe, but damned inconvenient for users.