Damian sells his soul to protect Gotham city from the apocalypse.
Leaving aside the fact that writers have the power to write any futuire they want. Jason, Tim and Damian can be written to be great Batmen and Dick can be written to retire happily. No one has to be a **** so that argument is pretty much nonsense. It's make believe. The writers make it up as the go along.
In Batman beyond Rebirth we learn from Bruce that Damian grew up to become the greatest batman he has ever seen. better even than him in his prime before he put on the faulty batsuit that sent him crazy [the same batsuit nearly turned Terry into a killer]
Something that's never been acknowledged for any other Robin.
Dick doesn't need to be batman. Dick isn't and is never going to be heir to the cowl because Terry Mcginnis is the only other Batman. everyone one else is just a temp. That is what makes the whole DC and Fans pitching Dick as the heir so sad.
So like i said the writer makes it up.