I don't think it's the load on Gmail that's an issue. I'd point more to Google Drive, Docs and the underlying shared storage infrastructure. Also keep in mind most of those 1.5 billion users won't be very active - a few million users that have no usage at all for a few months and then all come back to being extremely active within a few days can be pretty disruptive!

IMO it's not really about having the resources to scale, but the unpredictable emergent behaviours which can happen when the load profile suddenly changes