Prius owners walking out to find their cars without catalytic converters

I'm not a chemist, but I don't think the platinum catalyst gets used up. This site says a number of things can cause a converter to fail:

What Causes a Catalytic Converter to Fail?

It is important to note that, in many cases, a catalytic converter does not fail on its own. There are several factors that can cause it to malfunction. Here are some of the most common:
Incorrect air/fuel mixture

When the engine’s air/fuel mixture is thrown off, the converter may begin to overheat, come apart and eventually become restricted.

Pieces of the damaged converter may clog other exhaust components as well.
Contamination from fluids

Fluids, such as engine oil and coolant, that enter the exhaust stream can contaminate the catalytic converter.
car mechanic fixing a lifted car looking at catalytic converterIgnoring engine-related problems or skipping routine service can cause problems that contribute to catalytic converter failure.
Engine misfire

An engine misfire can create extreme temperatures inside the catalytic converter, causing the device to melt.
Engine overdue for service

Ignoring engine-related problems or skipping routine service can cause the problems outlined above, all of which can contribute to catalytic converter failure.
Impact damage

Impact from driving over debris on the road can crush the converter, damaging it internally. Damage to the structure of the catalytic converter can also be caused by rust and corrosion.
Excessive idle time

In extreme cases where the vehicle idles nearly all of the time, the catalytic converter may run hotter than usual, eventually leading to its early demise.