I agree with what you are saying in essence. Retailing can be inefficient. But with going digital you are also missing many features that you'd otherwise be getting - i.e. property rights. Having the option to dispose of your property at will. I bought Arcania from Steam and now I'm stuck with it. Had I bought the retail copy, I would have the option of disposing of it (not that I would - too much hassle). And let's not forget that virtually no-one makes demos these days and gameplay footage is extremely restricted.
As far as Indie games go, Steam's been fantastic at promoting them for the reasons that you already mentioned. But that doesn't mean that Steam (or other DDs) should be your only option. I wonder how many people have not bought Indie games because they are with DRM. Yeah it makes little sense to release a single bundle in retail form, but it would make sense to release a number of Indie bundles on DVD for reasonable prices.
Yes, PC gaming is substantially cheaper these days because of DD - you can offer 10+ different deals every week on a single platform, however this is due to the nature of the market. People get on their PCs every day and look for deals (thanks Bansama), but you wouldn't go to your local retailer every day to check for sales. And if you did, you'd realise that many of them offer much, much better deals than DDs (yes, I'm looking at you too, Mr. GOG. Especially GOGs can be found for much, much less in retail.)
Which leads to the foundations of my argument, namely monopoly costs and deadweight loss. A monopoly allows a particular publisher to sell their games for the highest price, leading to the highest possible profit (which they believe) but actually leading to deadweight loss. Ultimately, if the company had sold all of those units that were left unsold for less, they would have made less money (and satisfied far more consumers). The idea is that once you remove options, the company will no longer have an incentive to compete. How many years did it take for Valve to sell their games on a DD platform other than their own? Didn't it finally happen like last week? As competing DD platforms grow stronger and start catching up with Steam, their respective owners (or so to say, "AAA developers") will no longer have an incentive to offer their games on Steam, or other competing platforms. This will obviously allow them to price their games accordingly.
If we remove the retail, DDs can simply make themselves be the new "retail". All of a sudden their competition has been halved. What's their incentive for price competition? It's their way or the highway.
And let's have a look at the pricing of "AAA" titles right now.
Skyrim: 35 pounds. Same as at release (yeah I know it's on sale right now).
MW3: 40 pounds. Same as at release.
Anno 2070: 35 pounds.
SR3: 30 pounds.
These games are rather recent, I believe all of them came out in November.
DX:HR is 30 pounds. 25 Aug 2011.
MW2 is still 20 pounds (Nov 2009)
Oblivion GOTY is 20 pounds excluding today's deal. The GOTY was released on 16 June 2009 (lol). The game's like 6-7 years old?
Yes we get great deals on Steam but we can also get great deals at retail. The most important thing is price competition. Steam might be great for some (I love it), but it's ultimately a corporation with the goal of maximising its profits. Reduced competition is (almost) an overwhelmingly negative thing for consumers.
edit: oops what I wrote on deadweight loss isn't 100% correct, it's been a while. But the point stays regardless!