If I understand your question correctly you want to know if it’s really necessary to replace the sensor and if so, an explanation as to why.
The downstream sensor only monitors the performance of the cat converter by providing a second signal to the computer to compare with the upstream sensor (which DOES affect trim) and see if the converter is working properly.
Not fixing the probem won’t affect the fuel trim or the converter. However, it effectively eliminates the function of the CEL. If it’s always on, how will you know if a pending engine-destroying disaster is about to happen?