It depends on how many of the electrodes are affected.
For instance, your phone screen is probably expected to develop dead pixels over time, but there is a big difference between expecting a handful of pixels to stop working and expecting half of them to stop working. The former has virtually no effect, whereas the latter makes the phone unusable.
Likewise, the most important question for a brain interface is not “Are all the sensors working?”, it’s “Is the patient experiencing reduced performance?”