The problem is Classic realm population was created now and not in the past. Meta, game knowledge and player habits are different. People have been given few realms to choose from so it’s obvious most will pick realms with their faction in a better spot.
On vanilla pservers ~50/50 was achievable in terms of active players but that wasn’t 50/50 PvP participation as everyone was on one realm - pvp or not. TBC pservers do however have problems with high Horde population from what people were showing - and it comes to sometimes to add things like faster leveling ratio for Alliance characters and it still not enough.
So right now it’s quite hard to get any faction balance. Retail is a domino effect and Classic aside of it specifics will also follow with the faction tilt.