This is a good point. We're already seeing some of this, as many of the recent releases have been new enough to be available (hassle-free) on a number of download services already. And it's pretty clear from observing the top sellers list (over time) that these games are generally not selling as well as the older titles that are mostly exclusive to GOG.

I'm still not really convinced that GOG can make a profit long-term selling nothing but legacy titles, especially given the legal and technical work that apparently goes into their releases. GOG has no first-run, big-ticket titles to bulk up their revenue, and even their newest titles have already seen their initial big-sales period (if there was one). I'm sure GOG gets a shot of cash every release day, but selling Baldur's Gate 2 for $9.99 probably isn't going to bring in the same kind of money as selling Dragon Age 2 for $49.99. It makes sense for Goliath to pad its catalogue with legacy titles and long-tail revenue, but GOG depends on them exclusively.
Of course, I'd be delighted for someone to show me that I'm wrong.

As long as DD services keep trying to copy Steam, they'll ultimately fail. GoG succeeds, I think, because it's so different to Steam. No mandatory software, no DRM. Don't get me wrong, I don't mind that about Steam at all. But when you have multiple DD services that all have those requirements, it's a pain in the neck.

There's fierce competition between different DD services, but I think another, newer competitor will force several of them out; OnLive. DON'T HIT ME YET, HEAR ME OUT. While you and I don't much care for the surface, I have been observing the Arena from day to day, keeping an eye on how many people seem to be playing, and it looks like they have quite the group of people snatching up their games. People are attracted to OnLive like flies on shit (which is a metaphor that may have two meanings for some). I don't have an exact figure, but from what the OnLive people have publicized and from what I've seen popping on sporadically, they aren't going anywhere anytime soon. They're even expanding, adding games and servers due to demand.
This may be like an MMO, which starts out popular but slowly dies, but I think OnLive will always have a market for some. For us, who like running our games natively on our fire-breathing machines and take pride in owning our behemoth of a collection, the service holds no appeal. It kind of reminds me of the Apple market; many of us turn our noses up at the iPads and MacBooks of the world. But many mainstream consumers love them, and none of us seem to know why.

Greenhouse Games (from the Penny Arcade guys) is already become one of the first victims of this.

That's because their whole gimmick was instantly copied by the bigger names in DD.

The problem with GreenHouse is that they were too slow to expand their catalog and were charging too high a price compared to all the other options. They just weren't competitive enough. When was the last time they actually released a new game for example?
But the fact that they didn't take off doesn't mean other smaller ones will also fail. On that note, I think what happens with GreenManGaming will be of interest, especially with regards to their trading system.

Steam and Direct2Drive each have more Mac games than Greenhouse had Windows games, so it's no surprise they are doing badly. The problem is Greenhouse is a one-trick pony, and now that the big players have cloned it they are left with nothing. GOG's niche is DRM free games, but what does Greenhouse have? "Indie" games? All of those are on Steam.
If any DD service is going to fold in the near future, it's going to be Greenhouse.

Of course GOG will survive. Today's new games are tomorrow's old games.
And they're selling superior products (tested, compatible with the current OS, with no DRM that reduces the value of the game and with extras - some of which are actually sold separately elsewhere.

Goliath will not rule forever.
For we have David prepairing for his demise.

Forums have quirks. For instance, in the Steam forums, every time Valve so much as moves a rock in one of their games, vast fleets of waaaaaambulances are required to calm the whining down. Similarly, every time Steam is mentioned in this forum, people turn around and declare it PURE EEEEEEEVIL. The fact of the matter is that Steam (or anyone else for that matter) will never, ever be totally dominant in digital distribution, as you'll have to wait until the developers who run their own service (Telltale, EA, Stardock, Blizzard, indies, etc.) shut down theirs before Valve can afford to be PURE EEEEEEVIL for real and not starting losing customers to competitors (and even then, there's no guarantee someone won't start up their own DD service and start siphoning off your disgruntled customers *cough*Battle.net 2.0*cough*. You really can't afford to be PURE EEEEEEVIL in business, unless World of Warcraft makes up 60% of your revenue.)
Hmm... We will discuss this again in 12 months when the Founding Club 12 month subscription expires and everyone starts paying $15 a month (?) just for the privilege of access. Personally, I think OnLive has grossly overpriced its service and once the fees start hitting, I expect the customer count to drop.

While sales volume certainly counts, profit margin does too. How much does a company make per sale on, say, Jagged Alliance, versus how much they'd make on Dragon Age? I've got no idea myself, I'm not in the business. How much of that $49.99 goes to the publisher, and how much to the digital distributor? It could be that GOG makes $3 per title, and that's actually how much Steam or D2D make on their giant new releases as well. Although in Steam's case, all their Valve games are pure profit, since they own the publisher too.

More and more I am buying GOG, GG and Indie direct games only.
There are very limited options if one wants DRM free gaming. Steam is DRM and on top of that they keep publishers DRM intact.
Sales or not, I do not want my system to be burdened with DRM and I have long since given up on Steam and will never install it again.
When there is a deliberate attempt to block out competitors it is a no win situation for users in the end.
When there are very few competitors, there are very few reasons to give sales.

Actually, we'll probably be discussing this again in two years, because after the first year it's $5 a month. Because of that, I think that the price they're saying they will charge right now is just a placeholder.