if someone else can distribute more efficiently and cheaper - then they are up for it, no business wants the complexity of carrying out non core activities unless its forced to.
Yes, but why would they turn to Impulse? Steam:
1: Is far better established. Has been around since 2004, and all of the bugs have been worked out.
2: Has shipping games using its backend. No worries about games breaking due to poor-quality foreign code in their games.
3: Has a more stable distribution platform.
4: Has more intrusive DRM (for publishers, this is a plus).
Phase 3 provides Impulse with precious little that Steam and Steamworks doesn't already have. The only thing that might be worthwhile is the simplicity of submitting file updates; of course, none of us here know what the Steam process is like. Then again, major publishers don't care about something minor like the submission process; they're going to be methodical about releases anyway.
Phase 3 is fine for indie developers and so forth; the streamlined interface will help with that. It won't help major publishers.
What's most likely to happen is that Steam will be the place to go for the big-ticket games, and Impulse will be for the lesser-known ones.
Now your post can be considered false marketing, had it come from Valve, now; it's just false.
1: Nope, still got a load of bugs (several came into light with the recent releases of DoWII and Empires, such as f-ed up release date checks, downloading->patching->downloading (rinse and repeat) bug, etc. etc.) so Steam is faaar from bug free, and just about equal in that matter compared to several other digital distribution systems, such as Impulse.
2: You gotta be kidding me, sure Valve's own games are optimized for use with the Steam framework, but the fact that more than 75% of the games available on Steam are non-Valve games there is a lot of 3rd party code at work. (Also, take a look in the steamapps folder of your Steam installation folder, there you'll see that the file structure and workings of several of the games available via Steam is just like any other release, Steam is just used for DRM, downloading and patching of the game.
3: Stable distribution? Yeeeah right... With Steam servers crashing and extremely low speeds and halting downloads reported throughout the world on even the Empire: Total War DEMO release this is just not true.
4: Well yes, the Steam DRM system is intrusive, and this is of course both good and bad. It prevents pirating of games, to some degree, which is good. (While still not, in many cases anyway, screwing up your system, Starforce style.) Although there is a drawback to this; the Steam framework has to be running while running your game (as the Framework works today anyway), and no matter how you look at it; this requires system resources, more than would be used if Steam would just do a DRM check (like the discrete copy protections/DRM systems of today, such as SecuROM and Safedisc) at the launch of the game, and then turn itself off.
Steam is just one digital distribution system, with several drawbacks. And if those aren't attended there is a large chance that the market may see other equally successful alternatives coming up in the future.