Correct me if I'm wrong, but didn't the Combine solve most of the population control problems by simply doing massive population relocations?
Yes, but that would have required having gotten the bulk of a given population under control already to be able to displace people. You can't really move people around en masse if you haven't squashed their resistance and broken the will of the people to make them subservient in the first place. And while I'm willing to grant a small amount of credence to "they just mass slaughtered the mudslimes with alien WMDs", that's numbers for a workforce population going to waste and the Combine seem to care a lot about using up every resource available on the planet where possible, even hostile human resources. If they could so easily squash a guerilla resistance, then we wouldn't have the resistance that we do in the game as it is, and mudslimes don't tend to fight open ground wars specifically because they'd get curbstomped easily.
I would be inlcined to believe the only reason the Combine go full "stomp the shit out of the human opposition" in the second half of Half-Life 2 onwards is specifically because of Gordon Freeman and the threat he poses (and Eli Vance to some extent, as Overwatch starts to audibly put some emphasis on him as well in Episode 2).
Now, "alien terraforming" due to displaced species caused by the mass of portal storms that apparently broke out around the world is definitely also a good possibility but, as sandniggerish as Islamic extremists can be, they are actually a resourceful people when they need to be. I mean, we saw that people more or less adapted to the antlions' presence already. Say one armed cell manages to take down an antlion guard, and figures out how to extract a pheropod. Dealing with antlions becomes a solved issue, and we've seen certain cells of the resistance have already accomplished as much.
Fuck it, give me Islamic vortigaunts. We need to go stupider and funnier.
There's a reason the population of City 17 speaks English, if I remember right, North Americans got dumped in Bulgaria for whatever reason.

From this 4 year old video.
Granted, this is also presented as a caveat:

But that is just another theory, and I would think it better to go with what Laidlaw claims.