Meaning its the single worst possible future for Batman. Tim failing means all the hopes Bruce put on the kid were betrayed. It has a deep meaning in the Batman ethos.
Jason and Damian turning evil doesnt mean much, they are halfway there as it is.
Also it has thematic resonance with the fact that the future of DC is always depicted as the darkest possible future. That its one step away from falling into darkness, but somehow the heroes keep saving the day, the keep postponing this future.