Without knowing what is actuallly wrong and what is involved in correcting that, it's a bit early to give up on it.
It might help us help you diagnose it if you can tell us what problems you were having with the bike, and for how long, before you sent the battery off (and why you sent the battery itself vs some other part).
My personal guess with no testing or other information other than the age of the bike, is that at that age the battery cells are aged out, even if not worn out**** so replacing the cells would probably fix that issue. (I honestly can't imagine a 15-year-old battery of this type that doesn't have any problems; even my automotive-quality EIG-cell pack of similar vintage is most definitely showing it's age).
If that's impractical, then as long as the system on this bike doesn't require communication with the original BMS (it might) then you could buy a modern battery of the right voltage and mount it on the frame or a rack (or even carry it in a backpack).
****with really old cells, they may read a normal voltage with no load, then sag in voltage at varying rates under load, causing the BMS to detect overdischarge and then shut down the battery. Some BMS are designed with a safety feature so that when a cell voltage goes below a certain point, it "bricks" the battery by permanently turning the input and output off, so it cannot be charged or used, to prevent cell damage taht can lead to a fire. It's more likely to find this design in brand-name prebuilt bikes, as they are more likley to be held liable for fires traced to their devices, so making it harder to cause one with it is a good goal.