Well, I know my habits for DD purchases.
If it's a company that I have lots of loyalty for- which is pretty much Stardock and some Paradox stuff, and only their developed titles- I won't pay full price. I am more likely to buy DD, but mostly titles under $20.
I don't think Gamestop is what is keeping companies from jacking up prices, if anything, according to Stardock, it's the opposite. I know in the case of GalCiv II, they specifically mentioned that they wanted to sell it $10 cheaper on Stardock Central (their precursor to Impulse), but the big chains threatened to not stock their product if they did so, as Stardock would be undercutting them. If Gamestop really wanted to push used games, they'd offer more then a $5 discount, that's not enough for me to take the risk on a scratched disc unless I have no other choice.
Note: Stardock is one of those companies that understands the difference between total profit and profit margin. Brad Wardell even posted a rather arrgoant, though true, piece about a year ago calling some other developers "rockstars who didn't know how to make a profit", while crowing how GC2 made an eight-digit profit.
BTW we aren't getting the manuals and stuff, but what if I don't want a manual? I do think DD, legal or illegal, is what has kept PC gaming from going up in price like console gaming.
As for DD games and localization, some DD games are still region-locked. Stardock makes a point of not putting games on Impulse that aren't avaliable in the US, but will region lock to outside places, I've seen some games banned to Germans. Paradox region-locks with glee, a lot of their stuff isn't purchaseable by Americans- would a proxy work in this case?
One last thing, as for the revenue slashing increasing profit margins. It does- once you've gotten all the people willing to pay full price. Stardock here says they have a policy on this- they tend to only do two types of cuts: A very small cut if they think a game is still "hot", like $5 or so, or a very large cut to do the L4D scenario.
That said, the real question is: Is Gamestop necessary, and if so, for how long? Ultimately, I think the answer to that won't depend on any of this, but the direction of broadband in America- and whether Time Warner can get its broadband caps accepted. They're going to put millions into outing Eric Massa from Congress (who is their big enemy)- and they'll likely succeed since Massa is going against the tides so much. This is what is going to be the big factor. If caps put an tax on DD, then Gamestop's 100% safe.