Superconductors aren't the limiting factor for that, on paper we can already make a 40,000 km long 0.5Ω power line for costs comparable to current annual fossil fuel mining (Chinese annual coal alone is expensive enough for the aluminium).

The problem is geopolitics and that it's a megaproject.

It will highlight many of the engineering issues and some of the political ones, and ideally will seem like an overpriced mess when we reach the level where we even want to optimise design and process for a full-size grid.

For a true planet scale grid, think half a trillion dollars of aluminium, 3.75 years of current annual worldwide production: its doable, just not what you should jump into without smaller scale experiments.

I suspect battery storage will reach that point before any nuclear plant started in the West today generates its first watt: about 10-15 years.