I really don't think the brass case does much to contain the pressure at all. That is why some people make such a big deal about fully supported chambers.
If it doesn't support the pressure, then how can we possibly have been using the chambers that haven't been fully supported?
When a round is fired, the brass case expands and it retains all of the expanding gasses inside the brass case that is supported by a wall of steel all around. Unless you far exceed the suggested pressure levels.
If a brass rifle round fails and releases pressure it will quite often be because the primer pocket failed, because the pressure was so high that it could even expand the heavy brass case head, and as the case head expanded, the gas leaked through the opening.
I've had a lot of pistol brass split and the gun was not harmed. Where was that gas going to go?
That heavy steel barrel, even on tiny, cardboard thin cylinder walls, there is still more than enough to support the pressure of a normal round.
The steel couldn't care less about what the brass does, the only reason the brass is there is to hold the gases in place. The pressure required to blow the half inch thick chamber made of modern steel is far beyond what we expect brass to hold. We only care about the brass because it completely seals breech to barrel, and it is a whole lot simpler than stuffing loose powder and bullet into a firearm. It is convenient to use and it serves the purpose of a proper seal.
So, as was pointed out, why would we care if the brass was used, rather than a solid propellant with no metal? The only reason we would want to do away with brass would be for the weight, the expense, or the cost of the cases. The costs of production would be far lower than with brass if you could just squirt a plastic rod out and put a bullet into it, but that hasn't proven even remotely feasible.
Let's talk about the military. They have no reason to go with caseless, especially if there is even a slight chance that reliability would be compromised. Until you are dealing with hands on transport, such as a soldier in the field, weight is irrelevant. Until that is developed fully into a system that will function as well as or better than the current system, the army won't touch it. then, consider what it takes for the military to change this system. Years of gobble gobble, years of testing, enormous costs.
Civilian use? forget about that. Only the most dedicated of people will buy a completely new rifle just so he can use completely new ammo UNLESS there is an ENORMOUS benefit to it. Police use would be even harder than military procurement.
If the current production and purchasing system thought that it would work, there would be huge efforts in place to create this new system.