I've been cut off. And I have seen someone beside me get cut off. And I have seen FAs over serve people, myself included. The truth is an FA will be able to spot signs of intoxication before anyone else. Regardless, the FA has the authority. Of course, if the OP didn't like the situation, I suggest he get up and leave. Watch that first step.

Just like a bar, there is an issue of liability. Get off your plane, get into your car, and get into an accident, and the lawyers will come after Delta.