All I can do is offer some experience here. My sister had an old land rover and a cat went bad. Of course the dealer wanted some ungodly amount to fix it. Their “fix” was to replace the entire mid pipe (a whole new cat system). Thousands of $.
I took her to a local non chain muffler shop and had them hack out the bad cat and weld in a new one. Cost a little over $100.
My mustang had one go bad in the high performance mid pipe I put on. Sounded like a rattly coke can. No check engine light. I bought a new one and put it in myself (the pipe was modular and used clamps). Problem solved.
There seems to be several people on here that are being taken by their mechanics big time for cat problems.
Granted if one side fails the other certainly can go bad down the road and or even the replacement itself if its a cheap one. But for gods sake dont spend $4500. Thats silly