I think they are holding out on RDF to release it with the next run of classic servers because they know there is an audience for it and it will cause them to resub.
The business models for “classic” servers in other mmorpgs revolves around constantly re-releasing new “fresh” servers to simulate the game’s initial lifespan. These servers often have a theme or a new ruleset to shake up the norm and entice players to come back and try. Everquest does this with its “Time Locked Progression” servers and has been surviving off them for years now. One of their more recent servers changed how loot tables worked and it was a huge success.
WoW has already tried this with SoM so thats why I think they will bring RDF in with a future set of servers. Classic wotlk is going to play out without it and we may or may not get a continuation with Cata but we will definitely see a reboot with new classic servers around the time ICC is about to drop.