There's a GDC talk about this (up on YouTube, can't remember the title) about the cost/difficulty going alone vs going through steam.

The tldr is that valve apparently knows exactly how much it costs you to run this apparatus yourself, and the rev share reflects that reality. The numbers are apparently really close.

The catch is that as you sell more units, the cost of the apparatus starts to diminish as a percentage of revenue, so running your own marketplace starrs to make sense when you get into the blockbuster territory.

This pricing update apparently takes that fact into account, which means that when companies start running the numbers on their new game launch, the justification for building their own marketplace starts to go away.