I think there are good reasons to choose DynamoDB over a RDBMS that have nothing to do with scalability.

I've used DynamoDB several times over the past several years in the context of providing a datastore for a microservice. In all cases it was cheaper and easier than RDS, and the ability to add GSIs has enabled me to adapt to all of the new access patterns I've had to deal with.

For us, DynamoDB has become a 'boring' option.