It becomes more painful the more parameters the parent has- parameters which the dataclass may not even care about. It not only makes the class definition long, it adds so these additional parameters to the init signature, which is icky for introspection and discoverability. Lots of "What the heck is this parameter doing here?" head scratching for future me (because I forget everything).

I think that’s backward. The signature is there for the user of the dataclass, not the implementer. And the user had better care about that x argument, because it’s a mandatory parameter of the X class, so if they don’t pass one, they’re going to get an exception from inside some class they never heard of. So having x show up in the signature would be helpful for introspection and discovery, not harmful. It makes your users ask “What the heck is the x parameter doing here?” but that’s a question that they’d better have an answer to or they can’t construct a Y instance. (And notice that the X doesn’t take or pass along *args, so if the Y claims to take *args as well as **kwargs, that’s even more misleading, because passing any extra positional args to the constructor will also raise.) And that’s as true for tools as for human readers—an IDE auto-completing the parameters of Y(…) should be prompting you for an x; a static analyzer should be catching that you forgot to pass as x; etc.