I've no idea why it's delayed, nor how they produced this stuff - although it's based on a Manga series right? Usually they tend to actually stick more to those plots. If this OVA is just a plot from that, well, it fits in about as well as the other entirely separate episodes in the main series (where you can tell "That's the end of this one, next episode we do something entirely different for half a dozen!"

. Still a shame it's not more then one plot, but oh well, movie-length is sometimes better. Does depend I guess.
In any case, yeah, some series are fine to just end. No remakes please (god no to the
Cowboy Bebop live action thing...gah!).