> which just never happens. So that nonsense aside, I honestly do not know what the opposition to at-will employment is.
You don't understand it because you have declared that the argument of the other side to "never happen" and declared it "nonsense" to begin with.
But if you think it never happens, you are being naive and wrong. I have over the years had first hand knowledge of plenty of people being fired for reasons which have nothing to do with work performance at multiple companies. I've managed to stop some other attempts in the companies I've worked at, others were entirely out of my control to do anything about.
> I believe if you're bad at your job, your boss should be able to fire you. That day.
Even if we were to stipulate that no boss ever abuses this to fire you for unrelated reasons, this is also a strange attitude to me.
If you're bad at your job, your boss did a bad job interviewing you for the role and/or did a bad job providing adequate training. Also, if an employee is that bad at hiring the right people, I have little reason to trust that they are not also bad at ensuring they only fire people that are objectively bad at their job.
A process that does not give the employee some degree of recourse to argue their case and/or try to redeem themselves seems to me to be inherently one sided, unfair and abusive.
That does not mean I believe that an employee should be allowed to coast forever, but there is a massive gap from firing instantly to letting some coast forever. Such as e.g. the typical UK process, which usually involves raising a concern and formally drawing up a plan for improving performance together with the employee, followed by warnings if targets are not met, and finally termination. Yes, some people takes the piss and uses this to collect a paycheck a bit longer, others take it seriously and improves and become valuable employees; others will decide to leave on their own accord once they realise they will fail to meet requirements.