Any amount of cross-section of copper though is not - you take losses at (I^2)*R. You lose power as a square of the current.
There is an enormous difference between using superconductors at high currents and using any normal material.
Obviously the impact of this depends on what the critical current of a hypothetical room-temperature superconductor ends up being...but REBCO tapes achieve current densities of >40,000A/mm2 (at 77K). Depending on what you end up with, the expense and danger of maintaining the high voltage infrastructure could easily be seen as not worth it - particularly if it speeds up the ability to build out and maintain power lines.
