1) Even if US shale oil got to 10 million BPD - it does not replace Saudi Arabia (10m BPD) Russia (10.8m BPD) Iran (pre embargo 1.5m BPD) Venezuela (2m BPD).
Accordingly, If the above countries decide they need to change the status quo of cheap oil in order to maintain themselves social/ economically they can do so and Shale oil cannot pick up the slack.
It is also important to remember for the USA it is of their best interest not to completely crush oil producing countries. As the world ruler (hegemon) the USA benefits when there is less macro civil disruption.
Currently, oil producing countries need to build the current oil price to something much higher then it currently is (and quickly) in order to ensure the civil safety of their countries.
Could you imagine what would happen to the Middle East alone if Saudi Arabia, Iran, Iraq and neighbouring Libya and Russia all bankrupted due to economic bankruptcy.
That's why oil cannot sit at $45-50 for long - it is not worthwhile for anyone.