Gee, did we ever see a non-corrupt admiral in Trek? Dougherty, Leyton, Cartwright, Kennelly. Picard was right to turn down the promotion. Being an admiral would have made him a conspiratorial nutcase!
Agreed, even Kirk wasn't stupid enough to stay an Admiral for too long!
Wait a second, Admiral Kirk shoved aside Captain Decker for his own ego, and participated in the creation of an illegal device of mass destruction. He stole a starship too.