You have to remember the game was made back in 2004 so its very outdated. This game was made when Windows XP was still a new system. In that time, they have updated pretty much everything in the game and fixed many security flaws. Going back to this old core of the game is extremely risky because its very outdated and they will need to go back through every patch and add these security patches into the game before it can be released. Blizzard would not release a version of the game that is not secure. Wont happen. This would be very time consuming going back through all the code of the game. I can't see Classic coming out until late 2019.
An example of how outdated the system is: When TBC patch came out, EVERYONE had their friends lists wiped. This was because the code of the game needed to be updated to add the new expansion. This flaw was fixed and it has never happened again but this flaw would still be current in vanilla and need to be fixed before its released. I can just imagine the backlash if this happened in the current game and everyone lost their friends list.