Yes, but not for high temp superconductors. All known high-temperature superconductors are Type II superconductors in which vortices form in the electron superfluid, which gives it a very small resistance. (Small enough to be negligible by any real-world metric, but important for laboratory results.) There is also some resistive loss associated with the typical inductive/capacitive reactance when used as a transmission line, I don't see how superconducting lines can escape that.
Proper superconductors don't suffer the vortices. Lab experiments on superconductors suggest a current induced in a superconductor will continue on for 100,000 years. Calculations for some are in the tens of billions of years: if you get the current flowing, it will continue until the end of the universe.
What about when, inevitably, the cooling system fails somewhere in the grid? Disaster, or easily remedied maintenance?
At this point, I don't see it anywhere close to cost effective. But with, what, 10% transmission line loss and fuel prices the way they are... maybe a few $billion is a drop in the bucket?
If coolant is lost and the critical temperature is exceeded, the superconductor ceases to superconduct and becomes a fuse that will very quickly blow. I'd imagine the damage would be isolated to a short section of line.
