The need is 3m^2 cross section of conductor for the entire planet's current electricity usage, which at antipodal scale is 17 years of current global production of copper and aluminium. Production of aluminium is limited by electrical supply, and PV electricity is cheap enough to expand that aluminium supply. A 25% increase in global aluminium supply is significant, but not pipe-dream, and that percentage is sufficient for completing the task before the end of the century.

What I don't know is how long such a cable would last, which matters for total lifetime cost. But construction cost at current prices is on the order of a few trillion, which sure, is loads, but that's for a genuinely global infrastructure project that essentially replaces the multi trillion dollar per year fossil electricity industry.

Biggest problem is probably the geopolitics.