That decision might prove in hindsight to be incorrect, based on new information or a fresh perspective. The judge cannot be held accountable for that.
We accept this as part of the job, that difficult decisions cannot always be made perfectly within any legal framework, but still need to be decided.
If this results in disproportionate punishment, the system is at fault - the system of laws - not the judge, who was doing their job.
The judge's decision might alternatively prove to be corrupt, in that the judge has been influenced by something outside of their training, knowledge of the law, and the evidence. The judge should be held accountable for that.
Well, I already covered that in another comment: we could still hold them accountable for decisions deemed later to be incorrect with the same data available at the time.
In common law systems (US, UK for example) the trial proceedings is adversarial system. If someone should lose money because the innocent is convicted, it should be the defense lawyer. The trial is a game.
In civil law jurisdictions (like in Continental Europe) the trial is fact-finding public inquiry. The court is more actively involved in investigating the the case.
Well, this "best" should be judged and rewarded or punished too.
In order to express a desire for a specific change, you need to have normative ideas. At least a conception of how things should be (or shouldn't). So it comes down to, do you agree that judges should not just handle out death sentences, for starters, without any negative consequence on them if they get them wrong?
But note that if my request was normative (it was), the response: "Because their 'performance' is to perform their part best they can" is even less than a normative statement: it's a tautology.
There are tons of ways to perform badly without misconduct. Companies punish/fire/demote/etc employees who underperform all the time, for example. Or who make bad decisions consistently. Or even a single costly bad decision.
Why should "life of death judging" officials, highly paid at that, get a free pass?