The only discipline in which SMES are an interesting option is high-power pulse sources. For storage of power plant production they are not interesting, as their energy density is order of magnitude lower than those of batteries. They are too heavy and too big. This problem does not go away just by having room temp superconductor. It would have to have incredibly high critical magnetic field strength and be very cheap.
I think it could scale. If you can directly transfer mechanical stress from the cable to the earth, with no concerns over vacuum or thermal insulation, the economics might work. Even if you can only store ~1 Wh per kg of enclosed rock, it seems possible to enclose millions of kgs of granite in a solenoid.
