system
Given the following post, respond as if you're commenting on Reddit:
human
[Half Life]So what happens after humanity beats the Combine?
gpt
Well, after the Combine Citadel was destroyed, they had no means of retreating or calling in reinforcements. This put them into a corner, which was further reinforced when the portal storm was shut down by the Black Mesa satellite network.

We don't know how many Combine there are, nor how distributed their forces are across the globe. However, I'd say it's safe to say that humanity has a fighting chance (the slug-things might be a challenge, but the Vortigaunts are adept at handling them, it seems).

What we have, after the Combine are defeated and purged from the world, is a very post-apocalyptic scenario. The Combine were in charge of food distribution, which is now gone. They were also in the process of terraforming Earth when they were defeated, and atmospheric conditions have changed for the worse due to it.

Further, the chaos of the initial Black Mesa Event led to alien flora and fauna spreading across the world - many of the planet's native species are extinct and replaced with Xenlike ecosystems, such as the headcrab and antlion (and Antlions are prodigious tunnelers and will not be killed easily, as witnessed during the chain of events leading to the collapse of the portal storm).

We also have to deal with restoring crumbling infrastructure: The Combine were using cities as ghettoes (evidenced by the *only* population outside City 17 being rebel cells), and did the bare minimum to keep humans alive so as to recruit them into their own ranks. Much work will need to be done to restore cities to properly working order.

You also have the issue of predation. With the animal repulsion systems down for the count, antlions and headcrabs can now swarm the cities left standing. A coherent military presence will be required to keep people safe from marauding headcrabs, zombies, antlions, and more. The Vortigaunts might be able to help on this front.

So, to conclude:

* Food distribution is down for the count

*  There's crumbling infrastructure everywhere

* There's no coherent military with enough force to repel threats

* There's a global mass extinction in progress due to Xen fauna

* The planet itself is less liveable due to Combine terraforming

* Humanity is not connected - no means of city-city communication

I'd say humanity survives, but with millions if not billions dying as we revert to an agrarian subsistence civilization within walls to keep out headcrabs and the like. Rebuilding might take 1-200 years, and the planet will never be cleaned of Xen animals, leading to massive ecological changes. Isolated regions with energy, such as Aperture Labs (Chell might've popped out in a wheat field tended to by a nearby civilization) and the Black Mesa laboratory, as well as anywhere near hydro plants will become the new centers of civilization as people flock to these most developed areas. Kleiner and Freeman would likely become teachers for the next generation in an attempt to preserve as much knowledge as possible.