Doesn't iteration cause this strategy to be balanced out? After it becomes clear that two players have an unbreakable alliance, it's in the best interest of the rest of the players to destroy those two first in all future games.

This passage from the TV Tropes page on the Scrub is relevant:

The mistake the Scrub often makes is making up rules too soon. The Metagame can often turn an apparent imbalance on its head. A lower tier character can become a higher tier one, or vice versa. Or something that seemed initially very unbalanced can be countered with time and effort at learning the tactic. The Scrub circumvents this by simply banning the practice without making a good faith effort in actually getting around it with the in-game rules.