But there's still a major chokepoint: Apple's review process. Does Steam have a similar review/approval loop? I would guess not, since they don't allow just anyone to start selling on Steam like the "mainstream" app stores do.

It used to be that Valve had to approve and push your update out for you, though they did this typically within the same day you sent it to them. Now, you press the big red button so to speak. This also means you can push out updates outside of Valve's business hours. Great for emergencies...

They also have branches, so that you can release betas for your users to opt-in and test.