Yes, but far into the future, when automation is extremely prevalent, it could be that most jobs will be those that create public goods, such as research. If this becomes the case, then what will exist is effectively a planned economy.
Either way, Say's law is not exactly right. There can very well be demand without supply. It's just that it will take some time for the supply to exist (for instance, I demand a good that will allow me to travel through air, land, water, and space that is fueled by hydrogen fusion; the supply, however, does not exist).