People also wrote unrealistic books about magicians flying through the air and scrying on each other with crystal balls. Yet we have planes and webcams.

Naturally, some of the ideas fiction holds are feasible. In order for your analogy to apply, however, we'd need a comprehensive run-down of how many and which fictional concepts have become feasible to date. I'd love to see some hard analysis across the span of human history. While I believe there is merit in nano-scale technology, I'm not holding my breath for femtoengineering. Nevertheless, if such things were as readily predictable as people seem to think, you have to ask why we don't have the technology already. The answer is that actually expressing our ideas onto physical reality is non-trivial, and by direct consequence, potentially non-viable.

The human mind is finite, and there are infinitely many possible concepts.

I need backing on both of these points. As far as I know, there isn't enough verified neuroscience to determine if our brains are conceptually limited in any way. Primarily because we don't actually know how abstract mental concepts map onto physical neurons. Even ignoring that (contrary to memetic citation) the brain does grow new neural cells and repair itself in adults, even if the number of neurons is finite, the number of and potential for connections between them is astronomical. We simply don't know the maximum conceptual complexity of the human brain.

As far as there being infinitely many concepts, "flying car" isn't terribly more complicated than "car" and "flying." Even if something in the far future is given a name other than "car," we can still grasp the concept of "transportation device," paired with any number of accessory concepts like, "cup holder," "flies," "transforms," "teleports," and so on. Maybe it's closer to a "suit" than anything we would currently call a "car;" some sort of "jetpack" or other. I'd need an expansion on "concept" before you could effectively communicate that concept-space is infinite. Countably infinite or uncountably infinite? All the formal math I'm aware of indicates that things like conceptual language are incomputable or give rise to paradoxes or some other such problem that would make "infinite" simply be inapplicable/nonsensical.