As someone who is playing Classic, I’m not playing it for the same reasons I’d play retail - Retail is new, and we can look forward to things we don’t know about. Classic is a known quantity, and while it’s enjoyable, isn’t going to keep peoples’ attention forever and knowing everything isn’t always fun.
My interest will die sometime in Wrath, but retail can keep me longer if they fix the things that are broken. I miss enchants, and gems, and glyphs, and static gear. I don’t like the RNG. I don’t like the slider for each piece of gear from LFR difficulty to Mythic difficulty.