think about it the film can never work without a third one, as there needs to be a war in the future, so someone is sent back to inpregnant sarah connor....... so if they stop the war in number 2, no one was sent back, as theres no war, bu that means john connor was never born, so he couldnt stop the war, or even order the soldier back in time to protect his mum...hope you know what i mean!!!!
plus even if they destroyed all the work and part of the terminator from terminator1, they didnt kill the people who where working on it ( i know they main creator died) but i mean they still have the memories of the "arm and chip" so they know they can be built, as in t2 they say cos they had seen the arm and chip they knew that it could be made...... so they will still try to make it, and eventually they will