In my professional life I've come across a handful of colleagues who were demoted when they couldn't handle a certain role, or who have committed various kinds of misconduct, yet remained at their companies just fine.
However, you have to consider the downsides of this: is it fair that my workload is increased, because someone is underperforming? I'm not talking about occasional spikes, this is systematic. What does the company communicate to its associates, when "accepting" someone like this? What about quality, how am I supposed to champion quality if the company accepts the low quality output (let's forget "low quantity" for a moment) from someone else?
There's always a downside, and the workplace is an open system. You can't just "target" the low-performer, everything you do will target everyone on the team. If you try to "reconciliation" measures (other roles, extra training, coaching, easing of tasks), you're implicitly communicating something to the other team members. If you do it repeatedly and long enough, you'll eventually get: a) frustration, resentment and feelings of unfairness in other associates; and b) diminishing commitment and quality standards (i.e. "why should I care for quality if it is not demanded from others?").
P.S. I also feel a kind of shock every time I hear/read someone talking "lightheartedly" about firing someone. I'm just trying to highlight the potential downside of "not firing".