fly-guy

Conventional boreholes are cased with steel and cement. Vitrified walls would theoretically save that material cost (if it actually worked)

Great start, but one important factor is missing from this analysis: The surface area (for heat exchange) of a closed-loop borehole is tiny compared to the surface area in a natural hydrothermal system. The walls of the borehole will cool off relatively quickly due to the low thermal conductivity of rock, and you'll soon be unable to make power. Conventional geothermal gets around this by pulling from very large reservoirs that include natural convection and huge fracture surface area.

This means that a closed loop system needs many many many miles of boreholes in order to last long enough to pay off.

Another thing that should be considered: how do you keep the hole open while drilling? At extreme P and T, the hole will close in on the drillstring. Also, fluids turn very nasty in those conditions. You'll need some unobtanium and some of Superman's suit material.

ENergy conversion equipment can't easily be re-purposed for geothermal. The temperatures are usually much lower, and the working fluids are often hydrocarbons instead of steam.