You're trying to apply rules for things like supercritical CO2, to supercritical steam, which is like comparing opening a bottle of shaken-up champagne to dropping a full vial of nitrogylcerine.
Yes, to a certain extent, when the release is controlled, a steam boiler operates exactly as you suggest. But if the pressure drop is too far, too fast, or goes on too long, too much of the supercritical water flashes to steam, a process which happens at the literal speed of a chemical explosion.
The moving water and steam bubble has mass and inertia- a "shockwave" very similar to that produced by a chemical explosion. That spikes the pressure to upwards of 20+ times that of the system's operating pressure, which of course is catastrophic.