The thing is that as soon as any graveyard strategy becomes more popular, people pack more graveyard hate and Dredge suffers because of that. Grafdigger's Cage is a very good card against Dredge.
But will people pack it into their sideboard? If graveyard based deck will become so dominant that people want 7-8 slots in the sideboard to hate on graveyards, chances are they will pick up a set of Surgical Extractions and Grafdigger's Cage. Snapcaster Mage may be a reason why people would do this.
If people only want to spend 4 slots on graveyard hate, they'll probably stick with Surgical Extraction since that's a more versatile graveyard hate card, as it's also very good against Punishing Fire, where Grafdigger's Cage doesn't do a thing.
So basically, if Grafdigger's Cage will find its way into sideboards, Dredge will have a problem I think, since I think it will be run next to a full set of Surgical Extractions.

