Do you think any indie developer has the means to achieve a lower cost to distribution and promotion if they try to sell and support the game themselves?

Valve solves many problems for developers and these problems aren't imaginary and free to resolve.

Not saying 30% is justified for all games, but if you want a quality title it's going to cost more than just development. Since the Unity debacle we've seen some developers even say openly that costs of promotion and support dwarf costs of development.