if that wasn't a problem for blizz, we would already have the second build of graphics engine for the game released more than a year ago, that uses a core profile of opengl version Apple started providing two years ago, don't you think?
They don't have to abandon the old graphics backend. They actually supported 2 separate ones for years.
When Lion was announced they said when they do add support for core profile, that they would keep in current backend as an option for people running an older OS.