Yes, in the short run, players of Heroes of Stalingrad get short-shrift from this decision. However, I've more than gotten my money's worth from HoS, so I have no complaints. I'll continue to play it until the Soviet module for the new game comes out.
In the long run, we're all better off. Having a core game, to which we can add any modules we desire, is much better in every way. Even for scenario designers, the new system will, I would think, contain units, etc, for the whole course of the war, rather than the limited time represented in HoS. Furthermore, my only complaint about the game has been the lack of random map scenarios [with purchaseable units]. The addition of this feature in the new system far outweighs any annoyance due to the lack of future development for HoS.
I certainly intend to support this new venture, if for no other reason than that the modules I want to see, e.g., the complete Soviet war, won't appear unless they are justified by sales of the initial game. The possibility of these future modules makes me more than happy about Mark's decision.