It is indeed for the courts to decide how to deploy judges. So if Labour are able to fix the system so that more violent criminals are being arrested and brought before the courts, the courts will have to deploy judges accordingly. In doing that they will have to prioritise, and it's not exactly difficult to work out that getting dangerous criminals off the street comes ahead of, say, commercial disputes (which the government wants to steer towards arbitration anyway).
A government can help that process by, for instance, incentivising more lawyers to go into legal aid work - which I suspect is on Starmer's agenda anyway, given his background. Goodness knows, current pay rates are so pathetic that it wouldn't be at all difficult to improve on them. It's not an immediate fix but, I ask again, is that really a reason to dismiss the whole thing out of hand?