In addition to life in prison there is another way to prevent possession. Execution. It is my belief this is less likely to fail than the life in prison option.

But, ignoring your minor oversight, in answer to your question of course you are right.

The legislature can prohibit the possession of certain items. Then the law enforcement and justice people can, if they catch them, punish the prohibited behavior. Hence, because of the threat of prosecution, there will be some reduction in the number of people that engage in that prohibited behavior. Some people will claim this is worthwhile. But is it?

What does it matter if they possess a gun? What really matters is that they don't commit violent crimes against anyone else. Yes, the possession of a gun could enable them to commit violent crimes they couldn't commit without a gun. But if they are willing to commit a violent crime they will surely not have any inhibition regarding the violation of a law against possession of a firearm. Hence, the law against their possession of a firearm only makes sense if there were a way to not just prohibit and punish the behavior but actually prevent the behavior. As of this date the only way I know of to actually prevent behaviors are also the same methods (confinement or execution) that work to prevent people from committing violent crimes. So what's the point of a law against possession?

An argument can be made that the law against possession makes it easier to put people back in prison because they have to possess the firearm for a greater period of time than the time they spend committing violent crimes. Hence their exposure to capture and conviction is greater with a law against possession. But if your objective with this possession law is merely to increase the odds someone will spend more time in prison then why let them out to begin with?