I slightly emphasized something in there. You may notice it.GW has historically never given DA their own units.
Now, maybe you want to actually put a time frame in there rather than leave it open to the whole history of the game. Hell that isn't even the extent of older unique units, Dark Angels also got a unique Dreadnought and a unique Land Speeder back in the day as well. I think you are over generalizing (and thus missing older historical trends that prove your overly general statement false) while also ignoring recent trends in model releases. Don't mean to pick on you, but considering the units that have gotten releases in the last few marine codices and how they completely bucked previous trends I figure Deathwing kits are pretty damn assured unless GW pulls a 180.
Either way Wolf Guard got unique terminator resculpts where there previously was only the metal ones. Grey knights got brand new plastics for their terminators. And of course there are numerous non-terminator unique units that previously didn't exist before that got all new kits sprinkled in to the releases of Space Marines, Space Wolves, Grey Knights and Blood Angels. The way I see it, the who upgrade sprue thing was rather a more short-lived trend and GW has obviously moved on since.
Either way, it's just our own opinions. But in the future you may want to be a bit less generalized and hyperbolic, and when someone spots an inconsistency in what you said and the reality of the situation you may want to accept you were wrong with a bit of grace and move on. We all get things wrong from time to time, no need to get defensive over it. Just accept it, learn from it and move on.