The ugly part of this is there are bad managers and bad employees so we don't know what each situation is. Or sometimes both parties are decent people but their personalities each have one too many spikes that sometimes mesh super well with people, sometimes mesh poorly.

I agree the ideal manager who is a good communicator with a genuinely incompetent employee ought to make a soft touch. If they're really bad, ideally let them know they can look for a job if they wish, offer a neutral recommendation, etc. Let them know a PIP might be coming etc since legal basis must be covered.

Problem with a poor manager, even with a soft touch, is maybe their expectations actually are bad (especially a pointy haired boss who doesn't have a conception of the labor needed). Maybe they have OCD and see all work as bad, in the same way someone might consider Scarlett Johansen extremely ugly because of that one single freckle ruining the whole thing. I've seen this too. That's all to say if a manager comes in with the sweetest gentlest informal approach and complains to Scarlett Johansen about how awful the freckle is, well, even the soft touch won't help much there.

I would imagine in most real world cases we can see both, and we can see mixes, where it's a so-so employee who might be great under a strong manager, but is weaker under a OCD manager who puts the magnifying glass on the weaknesses. And of course sometimes managers will put the magnifying glass on the weaknesses because they want to eliminate this employee for various external reasons.