I understand everyone’s choices here but Arslan is a hypocritical fool for punishing the Trapezunites. He wasn’t there for them, despite being their overlord. He didn’t protect them. To expect them to reform an army after they just did his job for him and took horrific casualties. When this blows up in his successors face I’ll enjoy it. And I’ll enjoy the Trapezunites getting revenge on Kartvelia even more. This betrayal deserves nothing else but a switch response that ends their independence
When someone feels invincible, thinks is in the right to act this way. Arslan wanted to bring Trebisund into heel, David had to comply - also because being backstabbed by Kartvelia. Anyway we already know David will manage to reverse the situation, we have just to see how.
Besides, Trebisund survived because alternated periods of indipendence with token vassalage - so what David did is something past rulers of the Empire did before. Of course, his priority is to let digest those humiliations to his people and keep the fort until would be time to strike back.