Well, since they WERE the actual IG army before the Cadian line was created.
Granted, that's like saying that the previous range of Dark Eldar should still be maintained.
The best answer, IMO, is that just like a new edition supplanted Catachans as the basic IG line with a new Cadians line, this new edition should supplant Cadians with a Greatcoat set (with different heads to allow them modeled as either Valhallans or DKoK). That would make gamers happy, and be good for GW's sales as even old time IG players happily buy the new line.