One can hope we get by with the current approach until superconducting lines have been sufficiently figured out (it seems they don't need to run the voltage as high due to current being somewhat easier to deal with than voltage and losses not depending (significantly) on actual current (so thicker cables don't loose as much of their current rating to thermal limits than with traditional cooper/aluminum)).

The installation cost is tends to be so much higher that the potentially lower maintenance costs due to less weather damage doesn't make underground lines lower cost in the two to five decade maturities of utility bonds used for financing.