It's interesting to note that the three recently announced titles for August (The Man Who Knew Too Little (1998), The Avengers (1998), and Rush Hour 2 (2001)) were all new release DVDs back in 1998 & 2001, respectively. Between these releases and the Warner Archive releases of Deathtrap (1982)...
That assumes a healthy amount of those people would pay for them again to upgrade to HD. I imagine Warner looked very carefully at the sales of the classic Gangster films to see if films from that era would sell again in a box set.
I think you may be over estimating the sales numbers of some of these Warner catalog releases. I bet they print just around 3000 of them (or perhaps less) for many of them without advertizing it as limited (in that they can do a new run if they get low on stock).
You say this as though preservation/restoration and home video releases have a 1-to-1 correlation. That's not the reality. All the major studios have preservation/restoration efforts that are not tied to video sales. Warner is actually one of the best at turning those efforts into home video...