Yes, it would become peaceful.
No, it would still be violent.
Matters of this region were and are being intervened too much.
I believe that if all outside countries let their allies go then the region will gradually stabilize, with nothing to raise hate against, terrorist groups world gradually lose power. There would be setbacks, but the region would eventually stabilize. It would help if the oil was fully depleted
It would be more peaceful, but violence will always lurk around the corner. Still doesn't justify any intervention, however.
It would be even more violent, the USA gives aid to israel, saudi arabia and egypt so abandoning them wouldn't be a good idea.
The region was a mess before we got there, it will still be a mess if we leave. We may have thrown a couple logs onto the fire, but to quote Billy Joel, "We didn't start the Fire".
At least we wouldn't be wasting our men and money trying to protect nations that do nothing for us.
The only reason our allies in the region would be peaceful would be that the radicals killed them all and moved on to other regions.