I read that GOG on Steam sales piece and I can't disagree that they're probably devaluing games. I'm not sure if that's Steam's fault or the publishers'. When 98% of games that get retail releases, even if only the console versions, are valued at the same high price the system is already broken. If Assassin's Creed Brotherhood and Duke Nukem Forever launch at the same price despite being quite far apart in terms of quality and amount of content the consumer already knows there's a problem.
I think that Steam sales began as a response to customers looking at the price of a lot of these games, laughing, and waiting. Plenty of people were happy to pay $60 for Skyrim at launch. Very few sale games were able to displace it from the #1 sale spot for the first few weeks that it was out.
It's the publishers. It's absolutely the publishers. It's easy for GOG to blame Steam but I guarantee you nothing goes on sale on Steam without permission from the publisher or developer (if indie). Also, I really don't see how this is that much different than PC games going in the bargain bin at brick and mortar stores after they have been out for a certain period of time. The only difference is that this is more high profile and access to it is easier and generally at a known set time, at least for the big sales.
Publishers view games as having a shelf life and after about 6 months they know that they have pretty much gotten everyone who is willing to pay full price for it to buy it and have to start putting it on sale. That's just the way it is.
The deeply discounted Steam sales are all for older titles. Generally speaking at least 6 months old and in many cases a year. New stuff that is selling well does not go on 75% off sales, hence there has not been one for say Skyrim.
Also, in many cases, particularly for daily deal sales, the price cuts really do help push sales of older or lesser known titles. Does anybody really think Vampire: The Masquerade Bloodlines was selling a ton of copies prior to the couple of sales they had it on?
Lastly, I think the indie bundles are far, far more damaging to PC gaming and the perceived value of games. You see things like '$5 an indie game? OUTRAGEOUS! I am waiting until it's in a pack of 6 indie games for $3.99!' I honestly do believe that the bulk of people buying AAA games for 75% off on Steam wouldn't buy them anyway at full price while I do believe many of the people buying indie bundles would
buy some of those games at full price if it weren't for the bundles.
I am sorry that GOG's grapes are sour but this situation was created by the nature of the business and not anything that Steam dictated. The Steam sales are an effect, not the cause.