Room temperature superconductors would not give us zero-cost energy any time soon. Even if one had a high enough critical current to be used in transmission lines (which is not a given), transmission losses are under 10% in modern grids.
Modern grids are designed to keep those losses down.
If the losses weren't a factor the grids would be designed differently. Very differently. As in, North Africa would be so full of solar panels the generating fields would be visible from space.