You can never go back -- well, unless it's via the Caverns of Time or unless Blizzard ever decides to open classic-era WoW servers. Of course, Blizzard has already given a thumbs down to the idea. Really. No, seriously. Even so, many players continue to keep the candle burning in hope of rekindling a classic, expansion-locked WoW server.
Before you start cynically smirking about rose-colored glasses, consider this: There really are good reasons to love classic World of Warcraft. This is the content that made us fall in love with the game. It's what melted our hearts for Azeroth, offering some of the best and most enjoyable fights in Azeroth. Plenty of players still enjoy the classic content via retro raiding and more casual trips back for transmog gear.
The question is, do you miss the classic game so much that you'd play on an expansion-locked classic server? Would you perhaps even pay a special fee to unlock or subscribe to that experience? Remember, there would be no new talent systems or gameplay improvements, no new content or leveling curves. You'd start out with nothing but the original World of Warcraft experience, unlocking each expansion in a realmwide effort over time. It would be all old school, all the way, baby. Sound like fun? Sound like a grind? Sound like a fun grind?