Unlike fusion, we know we can build airships, in fact, we already did. It is just that economically, it doesn't work. And even less so if you are using a scarce resource like helium as a lifting gas.

In a sense, it is even worse. At least, fusion is a hard engineering and physics problem with many unknowns, we know we can make progress, and we do, slowly. Airships are well understood, the physics has been known since Archimedes (ok, with a little bit of fluid dynamics), and on the engineering side of things, we already know how to make every single part, we might get some improvement: lighter envelopes, more efficient engines, etc... but no amount of engineering will change the fact that we will need a huge bag of gas to lift a comparatively small payload, slowly.