Then you have the privilege of having not been in independent game development before Steam then! Steam takes about 30% and that comes with little to no stipulations. You can go put your product on other platforms, you can generate steam keys and then sell those keys off platform (and take 100% of the revenue), and from the very first sale you get ~70% of the revenue.

In the age before Steam you had to approach a publisher to get your product out there. And in those days getting a 70% deal would have been amazing. Except back then 70% meant something rather different. The publisher was getting 70% and you received 30% in "royalties" (and generally substantially less than 30%). Oh and you received $0 in "royalties" until the publisher cleared 100% of their costs. And finally of course you had to relinquish nearly every right to your IP as well.

I wouldn't underestimate the value that Steam adds. They take care of all financial related issues including trust, facilitate 'advertising', provide a captive audience upwards of 125 million, take care of data hosting (which can be a very significant issue in large-size products), and more. I'm sure they could get by with a smaller margin, but at the same time I think 30% is a pretty reasonable rate for the value they add.