Valve and the other large platform holders know that if they abandon 30%, there is no going back. Over time that 'platform tax' will get pushed lower and lower, until it basically reaches the marginal cost of transacting the sale and bandwidth. They are trying to hold that off as long, long as possible. At this rate it could be 100 years though.
If we stick to software and not tangent to movies/tv (itunes), Apple have absolutely moved some on this issue, since 2016 the App Store now offers 15-85 revenue shares for subscriptions once the user has subscribed for over a year.
Maybe not 100 years but substantial time. More likely, we need a platform transition as an inflection point that creates oppty for new market entrants. Part of new entrant's disruptive value prop may be better rev share terms.