Seeing someone being kept around even though they perform badly can just also make you quit internally. I’d refuse to put in any decent work when it’s obviously possible to be kept around and paid while not doing anything productive. Performing well in such cases just means I’m doing the other’s job too and their salary should be mine on top of what I already earn.
Psychologically, seeing someone perform at a task worse than you can make you more motivated to perform that task well.
Seeing someone being kept around even though they perform badly can just also make you quit internally. I’d refuse to put in any decent work when it’s obviously possible to be kept around and paid while not doing anything productive. Performing well in such cases just means I’m doing the other’s job too and their salary should be mine on top of what I already earn.