Why would we move Subban to LHD? And acquiring Russell doesn't prevent us from acquiring a number 1 LHD given that is not his role. He's a third pairing or 7th d-man. You don't have to block a young guy since he can play the 7th d man role, and understand he's probably not likely to block a young guy from the role because Ty Smith plays his off hand and we don't really have any lefties ready to come up. If he's blocking anyone, its probably going to be Mueller and Mermis from re-signing which is perfectly fine.
There is a fairly considerable drop from 14 to 18 in talent. At 14, you could easily end up with Jarvis, Quinn, Askarov, etc. At 18, you're looking at Holloway and Perreault and those types.
Greene doesn't give you a slight boost from 18 to 14, which as I said, could result in a big shift in talent. Secondarily, Russell may actually be cheaper than Greene given that he's only 1.5 in real cash. If you don't want to block young kids, seems kind of weird to then advocate for re-signing Greene anyway.