Responsibilites of airline after "external" delay [closed]

In short: In EU, if a flight is delayed/cancelled that is NOT the airline's fault, to which extent are they responsible to minimize the effect on the passengers?

Concrete situation: Recently I had a flight from A through B to C. The departure of the A-B leg was delayed due to the late arrival of the plane, which would have made the connection impossible and so about an hour before the original departure from A, I was automatically rebooked to a later B-C flight of the same company, 5 hours later.

It turns out that the very first delay was due to Air Traffic control regulations, making the airline not responsible for the delay. However, I later found out that there would have been faster options from A to C (via other connection than B) within the same alliance, so one might argue that the delay was due to the bad rebooking, which was certainly within their control, and so they are responsible (should pay compensation, etc). Is this the case?