I wonder if there is a business/marketing (or technical) perspective as to why the game is listed under one name rather than two on Steam? I would imagine from a Steam search perspective, not having to start your review feedback from scratch could be beneficial to the Steam Store algorithms? It might make key redeeming easier? I'm presuming now that anyone who buys the Steam edition now gets both versions which I don't believe was the case with the D:OS:EE. (But I could be wrong)
I do feel that if they could, it would be beneficial to have classic as a optional DLC that can be uninstalled, but I would be interested to hear a response from someone who understands the Steam and GOG architecture as to the reasons why they felt supplying only the full package was the best course before .