Up-front cost of wind and solar are low and falling rapidly. Their ratio to operating cost is high just because operating cost is very close to zero.
Systems that depend on steam turbines, such as nukes and geo, have high recurring maintenance cost as the turbines must be overhauled regularly: superheated steam is very corrosive. Since this tech is mature, such costs are not decreasing. Tech that depends on them gets continually less competitive, and will until solar and wind costs bottom out.
Heat engines are a method to turn low-grade heat energy into high-grade electrical energy. You are always better off staying in the domain of high-grade electrical, mechanical, pneumatic, chemical, elastic, or gravitational energy, without a detour through heat.
The claim seems to be that future grid costs would be minimized by using new heat sources to drive existing turbines, which then don't need to be built. Unfortunately, making and operating these heat sources is itself expensive, and keeping turbines working is expensive.
Turbines, particularly combined-cycle gas turbines, have a definite place in the future grid, as backup for cheaper generation. A turbine used only on some days needs less maintenance, so the grid will be designed to avoid need to run them most of the time.
The problem with geothermal, as with nukes, has been that (whatever way you drill) it costs more to produce the power than solar or wind, and that cost is not falling, unlike solar or wind.
So, for it to be competitive, storage would need to be more expensive than anybody could reasonably expect.