While that is not literally false, it is incomplete and leads to false conclusions. I've seen indie game developers report that they also see revenue bumps from seeing their games go on sale that they would not otherwise see, so it's not just Steam profiting. I have no reason to believe that the movie studios would not also see non-trivial profits from scraping the long tail. Pretty much everybody seems to win on Steam with those prices, which is why they've been going on for so long.
I have also seen indie houses report that they discounted too far sometimes [1], but all in all what I can see is that the indie houses have found that time-based pricing works quite shockingly well. By "shockingly well" I do not merely mean a rhetorical flourish... I mean, I personally have been surprised at what they've reported.
Remember, steam sales are approved by the publisher because they make them more money.
And yes, Valve profits off you. But if you were the only game consumer there was, Steam would cease to exist.