Why does the happy ending have to involve him hanging up the cape and cowl?
I think too many people have decided that Batman's only true happiness will come as Bruce Wayne. That he must either succeed in his acknowledged-impossible goal of ridding Gotham of crime, or just learn to accept that he can't go on forever and stop putting on the costume.
There are plenty of ways for him to have a Good End that don't involve him closing up the cave or handing the utility belt over to someone else. None of them are feasible right now because for one, if he was still around he'd still need to keep being Batman and thus necessitate not having an end, and for two he was given a really poorly-written Bad End already.
The comic where Diana looks into the possible futures she and Bruce could have together, though ugly as sin in the art department, featured a lot of interesting possible futures together, many of which could be classified some flavor of a Good End, and almost none of which seemed to involve him just being good ol' Brucie Wayne. (Of course she then decided to dump him because none of these was a Super Mega Happy End, proving that being invulnerable doesn't mean that repeated blows to the head can't damage something in your brain.)
A happy ending for Batman and/or his Bruce Wayne side does not require the sacrifice of one or the other.