Don't confuse "polite and friendly" with "nice". And don't confuse "terseness" with "jerkness".
We do no one a kindness when we allow someone to continue to make mistakes that could be prevented. Sometimes, the kindness is letting them fail. To feel the weight of the mistake. So they gain the experience they need. So sometimes, we need to tell someone simply "No". And some people will call you a jerk for that.
On the other hand, after you've spent so many resources attempting to train someone, continued incompetence is not "being nice". Someone who is truly nice would see that the skill is beyond them and remove themselves. Only a jerk would insist on maintaining a position they've demonstrated a repeated inability to perform. And only a jerk would try keeping them there.
Incompetent but nice does not exist. Or, if it does, it does not exist for long. Either the person stops being incompetent or they stop being nice.
We do no one a kindness when we allow someone to continue to make mistakes that could be prevented. Sometimes, the kindness is letting them fail. To feel the weight of the mistake. So they gain the experience they need. So sometimes, we need to tell someone simply "No". And some people will call you a jerk for that.
On the other hand, after you've spent so many resources attempting to train someone, continued incompetence is not "being nice". Someone who is truly nice would see that the skill is beyond them and remove themselves. Only a jerk would insist on maintaining a position they've demonstrated a repeated inability to perform. And only a jerk would try keeping them there.
Incompetent but nice does not exist. Or, if it does, it does not exist for long. Either the person stops being incompetent or they stop being nice.