In this context the original discussion revolved around someone being fired for cause (extremely poor performance). That someone was a knowledge worker and fairly well compensated. There was also little reason to believe that person wouldn't have a new job by the time they walked out the door (job market being what it is).
Thus my goal was to argue for a reason why you should always pay severance even when none of those conditions you mentioned apply. Otherwise I agree with everything you wrote.
Even poor performance is the fault of both the company and the employee to some degree... unless something changed, the company hired them for something they weren't suited to do, or failed to develop them properly. And generally, other employees don't know exactly how much someone underperformed, so it's moot from their standpoint.
The fact they were "fairly well compensated" is irrelevant, by the way. Severance isn't paid because people were underpaid.
Thus my goal was to argue for a reason why you should always pay severance even when none of those conditions you mentioned apply. Otherwise I agree with everything you wrote.