Dick Grayson can’t be a convincing long term Batman because he doesn’t hate himself enough. There, I said it.
He kinda does tho? It just shows up differently than Bruce's.
Bruce is all depression and brooding. He bottles his emotional responses to an extreme, forcing himself into the position where he can't show anything. Then, in his own desperation to stick to his philosophy, he'll do something stupid and short sighted that hurts someone he cares about. Bruce's self hatred forces him to isolate himself mentally and suppress his emotions.
But Dick? Dick let's it all out. Man has insane anger issues. Like, enough it practically blinds him when it takes over. In the Night Terrors event, Dick's nightmare was being thrown in Arkham after killing Batman in a fit of rage. As much as he tries to keep Nightwing positive, his anger will always bleed into it. You piss him off, and it all comes loose. And on top of that, he tends to self flagulate. He'll isolate himself physically, ignoring everyone because he thinks he deserves it. He'll make himself work and be tortured by the bad guys so no one else has to do it. He'll destroy himself because he thinks it'll help someone else.
It's not that Bruce has worse mental health. He has the tendency to keep his emotions bottled up, while Dick will let them out.
