If you stop releasing new Duals, then the next upgrade step for all these people will be Quad, thus they'll by-pass the duals.
And a Quad is a much better upgrade than a dual, the total processing power is almost double.
You are way out there!
If you stop releasing dual cores now, the next upgrade path for most people would not be a quad but nothing, they would not upgrade, thus wasting a lot of the investments in new dual cores completely, and not giving the companies new R&D cash to get working on new architectures. Quad Cores will get cheaper over time, stopping dualcore production now would leave a bankrupt Intel/AMD (whoever was crazy enough to do it) and get this industry nowhere.

