I guess the advantage of a Batman Beyond movie is that if it fails, they can always go back and reboot real-Batman instead fairly quickly.
Let's see, in OTL Warner Bros made the decision to develop a Batman: Year One movie (directed by Darren Aronofsky, written by Frank Miller) instead of Batman Beyond in 2000. It was abandoned in mid-2002 in favour of a Batman vs Superman movie (directed by Wolfgang Petersen, written by Andrew Kevin Walker) – then when they decided to go back to the "origin story for Batman" idea in early 2003, they pretty much started from scratch.
So, the most likely times at which a Batman Beyond movie would come out is either June 2003 or June 2004, depending on how quickly they can make it (and assuming they don't cancel it in favour of Batman vs Superman). If it's June 2003, assuming no butterflies, its main competition that month would be 2 Fast 2 Furious, Dumb and Dumberer: When Harry Met Lloyd, Hollywood Homicide, Rugrats Go Wild, Alex & Emma, From Justin to Kelly, Ang Lee's Hulk, 28 Days Later and Charlie's Angels: Full Throttle. So it looks like the odds would be in its favour. (I have a hunch they'd release it on June 13th, the same day as Dumb and Dumberer, Hollywood Homicide and Rugrats Go Wild – it'd probably win the weekend.)
Another consequence of this is that JJ Abrams's Superman script has a much better chance of being made: it was also cancelled in favour of Batman vs Superman but was revived afterwards, but then when they hired Bryan Singer as director he insisted on changing everything and that's how Superman Returns was made instead.