It's not a closed system. The oil will continue to flow into the gulf until pipe is lowered to the dome. Even then, the system will continue to be open with the hope that enough oil can be pumped to avoid an overflow. I believe that the only force will be the suction created by removing oil at the top of the pipe. If they have a closed system, they would face the same gas issues that caused the original problem without the cut-off protection.
And by funnelling it and capturing it, it is removed from the GoM way before it has any chance of reaching the surface, loaded onto tankers, and sent to refineries. Problem solved until the relief well can be drilled.