Here's the problem: not only do you have to dissipate 200W+ for your pass element, you need to drive it ultra fast with an extremely fast analog circuit that can withstand the massive magnetic fields (which pretty much means you need a PCB).

Yeah and good luck driving a 200W linear element (if it even exists, lol) with a few op amps--the driver which should deliver a few amps into the gate/base of the pass element, which in and of itself is a pretty difficult challenge.