If they are, then we ought to be looking at why prison isn't reforming them, and whether these people can ever be safely let out. And we ought to build more prisons, because an increase in national population will mean a proportionate increase in offenders.
Because the criminal justice system never really reforms people, and is based on a misapprehension of what what human beings are actually like.
Until the relatively recent past, prison was mostly just a holding space for people waiting trial, or holding spaces for people who had been sentenced to death, only to have their sentences commuted. Until 1832 there was over two hundred capital crimes in Britain, so you could end up hanging from a rope for a wide variety of different offences. This was challenged by the prison reform movement, which was driven by religious faith; a lot of the early prison reformers were Quakers. What they were working towards was the redemption of prisoners in a Christian sense. Redemption is harder when people are dead, so the reformers were keeping people alive so that they could consider their activity and find religion. Prisons (and workhouses, and a lot of other institutions) in Britain during the nineteenth century were focused rather heavily on redeeming the souls of sinners.
As this form of redemption went out of fashion around the end of the nineteenth century it was reframed as a psychological transformation, but left largely unchanged. The notion of reform from this point onwards (and which remains largely present today) is that criminals are broken people, and we use prisons to fix them. This doesn't take into account the fact that criminals may actively and consciously be wanting to carry out criminal acts. A redemption arc may be an appropriate framing for a young person who does not fully understand the consequences of their actions, but may not represent the situation of the average criminal. Certainly, there are people who will be deterred from carrying out further crimes by their experience in prison, but recidivism rates show that this not a universal experience.
So what to do? Probably send some criminals to prison for longer, particular with certain types of crime. This is expensive, but remember that all crimes have costs, and, at the current time, an awful lot of these are borne by the public.