I don't think WoW sold as good as it did because of it's graphics, but rather because of it's appeal to every type of MMO player.
Especially for casuals (the majority of WoW players apparently), cause it's easy to get into, thanks to rested status you're even encouraged to go offline and the small instances can be cleared in one sitting.
I never played the first Everquest, started with DAoC and loved WoW when it came out.
Not because of the graphics, I was sick and tired of endless grinding and WoWs quests and fast leveling were like a breath of fresh air for me...
And tbh WoW only looks good because it uses cartoony graphics.
Although many games would have more replayability if the graphics changed to fit the times, I can't imagine game developers continually patching their games...
Granted, MMOs might benefit from this technique, but I doubt any game that doesn't cost anything to play (apart from the one time you buy it) would get this kind of treatment, as you would need coders everytime you upgrade the game, which would cost way too much, especially for SP-only games like OOT.