30% is the standard cut. Some have adjusted in light of EGS - Steam reduces its cut the more units are moved and Microsoft just reduced their Windows Store cut to match EGS. I'm not so sure about some of the smaller/other storefronts like GMG (GoG takes 30% as well last I checked), and all the platform holders take 30% as they are recouping some pretty big costs and making up for selling hardware at a loss.
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Absolutely, Valve does a great job of supporting Steam with new features for both developers and players. But that's in part in their own self-interest as well, they want to keep players purely within the Steam ecosystem as much as possible as they've spent years gamifying the entire storefront/purchasing of games (Trading cards, tons of "account cosmetics" etc.)
They're not doing all this to help developers alone, but to drive more sales which they benefit from.