I doubt WotC would publish a book unless they felt it was good and saleable, so the question for those with that discerning caveat might require a bit of conjecture in regard to how much they are likely to find to their own personal tastes.
In the case where future 4E book sales are significantly below WotC's desired profit levels, there's always the option of WotC just publishing new content as DDI only.
For example, if the sales of PHB3 and DMG3 are not to WotC's satisfaction in profit levels, they could always change course in late 2010 and publish stuff like the Shadow and Elemental power source classes as "DDI only" for subscribers.
Back in the 3.5E days, this option was not easily available at the time.