This exactly is why I know a lot of old Aion players like myself would come back. Retail is small because all of us are not playing it and never will. We don't like Retail Aion and what it has become. We'd love to play Classic Aion, though. Just like with WoW, it will largely be a separate community from Retail with little overlap in players. Also you'd be surprised how many players nowadays prefer paying a sub fee to playing F2P games. For many F2P and P2W games are a major turnoff.
I haven't played in years but I would 100% come back for an Aion Classic server and I would definitely pay a subscription fee. I think it would be a great idea and I really hope NCWest takes this seriously. It's a no brainer idea with very little work required to even implement. Count me in.