Right. There are similarities, as you said, to the concept of the old hero in a dystopian future and a general dark tone. But besides that, the stories are pretty different. Even the talking media heads which are used to compare Reign to DKR is executed differently--here they're basically spokespersons for the government, whereas in DKR they were used to show society's views on Batman.
If people want to a see real ripoff go look up Zinda and see how it compares to Oldboy.