As someone that has designed gas turbine engines, you really don't understand how they actually work. More load equates to more drag on the generator, which adds more drag to the turbine, requiring more fuel to keep it turning at the required speed. Just like an AC motor, more load, more drag, more current to maintain speed. Some power plants can adjust to the load very quickly, others slowly, but they can adjust. A 500MW power plant isn't sitting there pumping out 500MW regardless of demand and dumping excess to a resister bank when no is using the power.
You are right they have to have enough turbines spinning to meet the peak demand for a given time period and there are idling costs associated with that, but they are not binary off/on systems. We should be moving loads such as car charging to off peak times to help flatten the demand curve, if we actually cared about being green.
Regardless, miners are using green power that could be going to other purposes if they weren't using it. Think you can waste power just because it's renewable, if just like thing excessive waste is fine as long as it goes into a recycling bin or you can eat a cupcake because you ran a mile (there is a specific name for this, but I can't remember what it is). If we add 1GW of green power to the grid, but use it to justify increasing our demand by 500MW, we have only actually removed 500MW of fossil fuels, not 1000MW.