Seafoam isn't going to fix a cat. The purpose of a catalytic converter is to burn unused fuel, and it uses platinum coated "honeycomb" structure similar to what you see in a radiator. The platinum is the catalyst for burning the unused fuel, and the honeycomb structure maximizes the surface area of available platinum.
The primary reason that a cat needs replaces is because the platinum coating and/or honeycomb structure has been damaged by excessive heat, rust or exhaust residue leaving less surface area/platinum exposed to catalyze the burning of unused fuel.
This is why it really is important to fix things like O2 sensors and other ignition components when the dummy light comes on (and not just ignore and/or reset it) because when those things are broken, the vehicle can "run rich" which means there's excessive unused fuel in the exhaust which the catalytic converter is there to mitigate, but just like any other mechanical component the more its used, the faster it gets worn out. The reason catalytic converters are really expensive is because they, unavoidably, use platinum as a catalyst, and platinum is an extremely expensive material.
Seamfoam only "cleans" the internals of the engine, and with modern vehicles is more of a hail mary than a real solution because, as others have pointed out, typically while cleaning the inside of the engine it breaks off chunks of gunk that clog smaller oil passages which will lead to complete engine failure.
If your cat is dead on a 7-8 year old vehicle that you put 2,000 miles on a year, it's either a faulty cat, faulty code, or you have a serious emissions problem -- probably due to improper maintenance. If you have an emissions issue, you have to tackle the source: Broken ignition/timing components or a sludged engine. If you don't fix those things, a new cat will be destroyed in short order.
There is no easy fix-in-a-can to a sludged engine (e.g. seafoam) causing emissions problems. You'll have to strip the engine apart and manually clean it (if the sludge is bad enough). Just like most things, an ounce of prevention is worth a pound of cure. Use the proper oil for your vehicle, and change it according the the manufacturer's schedule. Even if you only drive 2,000mi a year, you need to be changing your oil according the the maximum time interval specified in your car manual. Oil, once exposed to engines contaminants (especially gasoline), begins to break down and continues to break down if even you aren't driving the vehicle. And, there IS a significant difference between oils -- the cheapest stuff is fools gold.