And once you're big enough, you can just offload your inefficiencies onto the consumer (the "enshittification" stage). As long as you're not _terribly_ bad, competitors won't be able to overcome your network effects.
What _really_ held companies from taking over the world was the inherent overheads of distributed corporate structures. Before computers, you couldn't really manage a horizontal company with offices in hundreds of different cities. Network effects were also less pronounced.
Internet-enabled companies like Google (or Yandex, Baidu, etc.) are a really great example. The early Internet was about as close to a perfect free market as possible. And in every case (Google in the US, Yandex in Russia, Baidu in China) the same thing happened: one large company ended up dominating and then gobbling up all the competitors.