If AWS were to go to a per-minute billing cycle, they would be instantly more price-competitive with Google's offering. Or, to put it the other way around, those leftover minutes form a significant chunk of AWS's profit margin.
For how long is the question. Historically, it’s been considered common knowledge (might just be an urban legend) that AWS, even if you pay for more traffic, at some point just throttles you, the same way that they do with IO.
The run of iperf refuted your refutation.
Though there would still be other things like the lower on-demand rates, custom shapes, networking that scales with shape (rather than being coarsely grouped), being able to attach SSD / GPUs semi-arbitrarily, and so on. For those that care, not having to pay up front for the best price is also a huge deal. You see the same thing in GCS vs S3 as well: Glacier and S3-IA have a few rounding up gotchas that catch many people out.
All that said, I hope we all get to per-minute billing.
Disclosure: I work on Google Cloud (but haven't talked to the Metamarkets folks)