Core infrastructure may or may not be low margin (traffic rates are nearly pure margin designed to create a moat and may or may not eventually be challenged by regulators as anti-competitive; it remains to be seen). But the lion's share of cloud margin comes from value-added managed services e.g. AWS Lambda and GCP PubSub, which are exceedingly more expensive than their underlying infrastructure but not nearly so expensive as to justify small companies hiring FTE to manage open-source equivalents. And there aren't nearly enough cloud players to justify competition building out competing implementations for, say, the Lambda API, with the sole exception coming to mind being competing object storage providers implementing the S3 API.

Multi-cloud isn't a myth, but it's pretty close. It requires lots of discipline that most customers won't have. The smart money is on cloud providers improving their margins, not shrinking them.