What simpler alternative is there? The OOP way would be to always hide data. This certainly has a different set of trade-offs, and sometimes requires the visitor pattern. Maintainable and simple are not words I would use to describe these cases.

The other alternative is to emulate sum types through run-time type information or using code generation like protobuf. I also fail to see how these approaches are simpler or more maintainable.

Sum types are not a panacea for all software development ills, but they're the simplest and most direct solution to a concrete problem: My data is either this type or that type.

Sum types are simpler, and harder to misuse, than the common alternatives (unions or dynamic typing).