First, Google doesn't need to prevail, it just needs to get a sizeable enough chunk of the market to make a profit. E.g. even if it's an eternal long-term 33% to AWS, Azure and GCP, it's a rational investment for Google. The market's big enough.

Second, Google is clearly betting that its scale will indeed give it a benefit -- the scale of its data centers which include all of Search and everything else (e.g. raw costs for computing and storage being cheaper than competitors), the scale of its investments in machine learning which it makes available on GCP, the scale of its investments in private networking and security, etc. Part of Google's pitch is that it's providing Google-class security and networking which AWS and Azure simply can't match.