This isn't an indictment of either Darqueze Dennard or Justin Gilbert. They're both fine players and worthy of first-round picks.
They just shouldn't be Detroit Lions.
The Lions have spent plenty of picks over the past few years adding young talent to the depth chart and cornerbacks take a ton of time to develop. Bringing in either one of these guys to start over Darius Slay and Bill Bentley will stunt their growth and essentially waste the investment made in the players on the current roster.
Plus, each flashed some signs of competency last year. They weren't consistent, but that's where the development comes in. A mature player consistently makes the right play, and you only become mature by experience.
Lastly, fans love to point to Chris Houston as a problem. He certainly wasn't good last year, but he has played well in the past and would still count for $5.2 million if cut. Therefore, there's a good chance he rebounds, and even if he doesn't, cutting him this year isn't an option.
The Lions have invested a lot of money and draft picks into the position. Pulling the plug now would be like buying a new car because your one-year-old vehicle got a flat tire.