I mean that does partially reduce the chances of a cartel, but not really near as likely as you think.

Most countries have a pretty strong ban on most kinds of weapons, the US is one of the few that lets everyone run around with their rooty tooty point and shooty, but most countries have implemented bans. Some because the government doesn't want the people having them, and in others the citizens call for the bans because they don't like the idea of getting shot by their fellow citizens.

It won't be long before citizens and governments get tired of models being used for criminal activities and will eventually lay down laws around this. Models will have to be registered and safety tested, strict criminal prosecution will happen if you don't. And the big model companies will back their favorite politicians to ensure this will happen to.

Now, that in general will be helpful as there will still be more models, but it will still not be a free for all.