I don't think this is true. If nominal-by-default is not on the road map, then most people never have to do anything. If I can generalize about myself at least, most data types that are defined do not have (additional, role-enforceable) invariants. They are mainly simple combinations of other types in ways that allow me to structure my code well. So the vast majority of types defined by people like me should have representational parameters, unless one of the constituent types is a Set or Map or what have you, in which case nominal can be inferred, assuming Set and Map are appropriately annotated. Nominal-by-default would require annotations for every minor definition like this in order to correctly specify the type.

By contrast, representational-by-default imposes some work/diligence on authors of data structures with invariants, which tend (in my experience) to be the minority of definitions, and typically come as libraries.

I feel a little bad suggesting 'unsafety' as the default, but the safe default imposes a significant, perpetual cost on everyone using the language in return for defense against occasional oversights by library authors. That doesn't seem like a good trade off.

Dan