From what I have seen and read in regard to this problem, it’s unfortunate that Toyota did not design in an overtemperature detection sensor or a coolant line flow sensor to shut down the inverter with converter before it overheats and consequently fails.
The ECM on a 1984 car that I bought new failed after the warranty ran out. As I recall, a new one was around $700 or $800 but the dealer of that brand got a rebuilt (refurb?) for me for around $300. Possibly the aftermarket will eventually get around to refurbishing Prius electronic parts.