There are about a zillion ways it could prove to be impractical. Apatite is a crystal, and presumably this lead apatite is also a crystal. We also don't know if it can be deposited in a useful thickness; the samples tested so far were created by gas deposition on glass. Can it be built up to a useful thickness, and maintain its superconducting properties? All unknown.
But, real progress always comes in small steps. It's exceedingly rare for any discovery to result in a useful product immediately.