It depends?

Last time I looked many of the alternatives didn't have as good support for some of the transaction-friendly things like conditional writes.

Hopefully if you're trying to build something complex on top of S3 and have any expectation of potentially needing to run on top of other infra in the future, you know which magic features you need and can see which of them are supported by what vendors/OSS projects. :)

But it's not really that different than doing cool stuff by pushing a bunch of things into Postgres or Kafka or [other thing] and not explaining how those things work.