The point here is the timing
Zhuge Ke lost his chance to win the battle when he gave the enemy 10 days, during this time, illness had took most of his soldiers and what he did? order a new attack on the city! and then, when he realized his mistake, he orders the retreat.
If he had retreated earlier, he could have ordered a more "safe" retreat, a gradual one and not just leaving soldiers dying on the road!
What is your source for Ding Feng getting arrogant? It isn't in the SGZ or ZZTJ that I can recall?
So he is a monster becuase you believe he got his tactics wrong? Making a misjudgement make one a monster? I'm not sure retreating after he discovered the trick would have saved many lives in the retreat and the need to retreat fast would still be there, there is no chance of really really slow retreat with heavy medical care+mass sanitation improvements that would be required to prevent the heavy casualties
You may be right, I think his increasing harshness near the end though suggests a man perhaps feeling some pressure or rattled a bit by defeat. In terms of Ke being the last of Wu's independent Cic's, it requires confident and trust between ruler, court and the commander to work. Trust in ability and loyalty to each other, that there is respect and so on. When Sun Quan was well and the court stable, this could happen but once he became elderly, unwell and his authority dimmed, that was no longer possible. The court was no longer stable, constant backfighting and infighting, Sun Quan and Sun Hao would become paranoid figures (not entirely without reason), Sun Xiu could have done it but never did. Zhuge Ke as the wonderboy with a child emperor was untouchable, he could do it becuase everybody respected him or felt they couldn't move against him. Once he lost, his authority, his sheen was gone, the unstable court could pounce for power
I think his fall was really bad for Wu, despite his capable successor, but more part of a disease that had already started before his time.
Ding Feng was trusted enough to be part of Sun Xiu's coup against his overpowerful boss but he was more a trusted senior general then ever a CiC figure like Zhuge Ke or Lu Kang.