If Paul hadn't have been so removed from the streets. Gotti never would have got the backing he needed from the other captains. Armone/Gallo, DeCicco, Gravano (who was not yet even a capo. He needed backing from these other guys, and the weight their crews carried, to bring every thing together once the shooting was over.
No way a single crew takes out the boss then stares down the rest of the family as well as the other 4 family's.
Paul has been creating the divide for years prior with his greed and holier than thou attitude. Gotti just was able to capitalise on it when the stars aligned.
IMO Castellano was a very shrewd and switched on operator and years before may have even been a formidable force on the street but in the end age and good living caught up with him. When you cant see the angles no more its time to get out cause the streets ALWAYS watching.
In the words of Carlito Brigante “you run out of steam. You can’t sprint all the way. You gotta stop sometime. You can’t buck it forever. It catches up to you. It gets you. You don’t get reformed, you just run out of wind.”