Well think about the last 10 years Steam was the only platform for most of the time & PC gaming exploded and was averaging around $30-32billion per year in sales according to the now defunct PC Gaming Alliance. So 30% of 10 x $30-32 billion is a massive amount of money! Then you could also argue between 2004-2009 Steam also made a lot of money! For what Valve got from PC gaming I do not see them giving much back even the used to be free Half Life 2 Nvidia sponsored demo became an item you had to buy eventually
How many games did Valve ever give away I am struggling to think of one. Even EA gave plenty of older games away! I just think Valve got caught being greedy & happy to rake in the easy money & allowed others to take a lot of their core business away now Epic are going aggressive to build their platform up means Valve are going to struggle to convince publishers who want to use Steam @ 30%.