No, it's actually true that Zerg gets to an unbeatable mix. The big problem is that they can tech switch so quickly while Terran and Protoss can't because they need the unit producing structures. Zerg builds one spire and all of a sudden can make a ton of Mutas in a short period of time. For a Terran to go air, they need to produce several starports to produce similar numbers. So if Terran tech switches to deal with Mutas (Thors/Marines), Zerg can quickly change to Infestors and Brood Lords, and all of a sudden Terran needs to mass up Ghosts, Vikings, and Marines. It's just impossible for Terran to keep up with how quickly Zerg transitions from one composition to another if you let them tech up too far.