Note that there is no guarantee that that would ever happen. Electrical resistance is not the only thing you need for something to be an economically efficient power line. While superconductors are by definition excellent in terms of electrical resistance, there is nothing to guarantee that they wouldn't be too brittle, or too heavy, or too hard to mould into the required shape, or simply require materials that are too rare on Earth. And all of these would not be things that can just be worked around with better production processes or smart engineering - they would be fundamental limitations of the specific material, just like the low temperature requirements of currently known superconductors will never be improved with more research.

So this isn't a matter of when they would reach the point of being better economically, it's also very much a matter of if they would ever reach that point. Hopefully, we'll get lucky one day and find a material that is superconducting at room temperature and above, that is study and light and easy to make into wires and made out of abundantly available elements. LK-99 certainly wasn't most of these things. Even if it had been superconducting, it wasn't a good candidate for any of the other properties we want anyway, so it likely wouldn't have been much better than other known materials for most applications.