My '11 Outback had its OEM battery for about 5 years until I allowed a friend with a battery tester show me that my battery was getting weak and I should replace it before winter set in. I did, but the car was sold at 10 years old and I never had any problems.
My '19 Crosstrek which was a Demo/courtesy vehicle prior to my purchasing, is still on it's OE battery with no problems, including a time when I accidentally bumped a dome light and killed the battery overnight. A jump, a dealer testing (I am very good friends with the service manager) to determine the battery was fine, and we are trucking along, just like the old Dead song.
I don't subscribe to the saw about OEM batteries being wholesale crap. Yes, there will be the few that fail prematurely but then again so will there be aftermarket batteries that will fail as well.
As for Costco and Wally World batteries, remember that those batteries, while showing a brand name, are built specifically for the stores in question to their pricing parameters. And that goes for anything else they and others like them sell. The brand may be a familiar name, but the quality is NOT exactly the same.
As for the OP and the problems with the dealer and supplying a replacement battery, I'd say that part of the problem is likely the dealership service department itself. And pinching a battery from another vehicle, even for the short term until a new one arrives is not only a good idea, its good will expression.
And a call to SOA will go a long way to get the problem solved too.