OK, I'm looking at it this way - Cass's reasons for quitting were, eh, a bit murky, but basically what it boils down to is 'now that Bruce is dead, I don't wanna be Batgirl anymore'. I'm guessing that once Bruce inevitably shows up again, that means she'll take the cowl back. Which means we'll have a few years of Steph as Batgirl, doing exactly the same things she did as Spoiler and getting yelled at just as much, then Bruce will return, Cass'll take back the mantle of the bat, and Steph'll go 'hey, might as well be Spoiler again', since in comic book time, this will have only taken about six months.