If I remember correctly, at the time Valve justified the 30% by pointing out that Apple was charging the same for music and video content. And Valve immediately started building value-added services like forums, updaters, multiplayer support, achievements, etc. to justify the price.
If you compare what Valve was doing to the physical media distribution methods of the period, it was a MASSIVE improvement. Back then, you could sell 10000 units to Ingram Micro or PC Mall, or whatever, and you only got paid if they sold. And any unsold inventory would be destroyed and the reseller would never pay for it. And if you actually wanted anything other than a single-line entry in their catalogs, you paid a promotional fee. Those video games featured with a standup display or a poster in the window at the computer store? None of that was free; the developer was nickeled and dimed for every moment their game was featured in any premium store space.