For Steam specifically, I can state with confidence that that's not true. On a phone at the moment so linking is difficult, but Jeff Vogel (the definitional "indie developer", there are very few people who have been doing it as long or as consistently) has a GDC talk on YouTube where he goes through his numbers--and he's a guy who went to Steam very late overall.

Steam's bigger marketplace and that it removes the uncertainty and risks around payment provider management (chargebacks etc.) easily make up for the bigger cut. Do less work, make more money. It's not unreasonable.