Even if Half-Life 2 ships with Vanilla Deathmatch (which it probably wont, considering the tight lid Valve is keeping on multiplayer), the game will still have mods.
So I don't see how Half-Life 2 is in any sort of danger in regards to multiplayer.
Edit:
Found the IGN post about Alien Slaves. I'm not sure if I believe it though, because there is no word about Alien Slaves in any other preview except IGN's.
Alien Slaves have not been confirmed by Valve.
To clear up the confusion:
Some idiot that wrote a preview for a gaming magazine on Half-Life 2 said Alien Slaves instead of Ant Lions. He said the player will "ally" with the Alien Slaves. What is actually true is that the player will "ally"...
Actually the physics are pretty good. For example, they show one of the soldiers falling and having his foot get stuck mid-way through the fall, then finally slamming into the ground. Looked very realistic.
When one of the Stalkers was shot, he fell to the ground and started squirming and...
As far as I know, there are a few various types of Assassins in Half-Life 2.
The assassin robot, the female assassin, and perhaps the prowler creature.
The prowler looks like it's an Assassin-class monster because it seems like it'll be very agile judging from the concept art...
Cool, this is related to Half-Life 2 I guess because Half-Life 2 has aliens and Alienware is a company with "alien" in the name just like how Half-Life 2 has aliens.
Wait, no.