My counter argument is that if the marketing message in the marketplace is that VFP is end-of-line (and that message has been floating around since VFP3 days) then what else could MS possibly expect but declining sales? If you don't promote a product, and moreover even allow negative marketing messages to prevail, then naturally sales will fall.

That may be. But this is also a chicken and egg argument. If the sales of new versions had been better, perhaps MS would have seen some value in producing a new version. However, I do believe that nothing was going to make their marketing of VFP any better.