The difference here is in cataclysm there was a shit ton of servers to choose from and overall population was healthy enough to have them all decently populated. Once population problems started happening? They introduced more and more cross-realm features to deal with it. As of retail right now, realms only matter for the most recent raid on Mythic Difficulty.

So what if Blizzard removes layering and adds more servers? Then you better be great at gambling, because 45 of the 50 servers will be dead after 1-6 months from all the tourists leaving. You spent weeks /played on your character? well congrats, you either need to buy a server transfer (if they exist), wait/hope that blizzard merges servers soon, reroll on another server (haha... yeah), or just quit yourself.

See the problem here? Buying a server transfer sucks and makes your realm identity get ruined. Waiting for servers to merge could be a few months and come with their own set of problems (naming, server identity issues yet again...). Rerolling on another server is likely the worst option and hopefully I don't need to explain why. Lastly, quiting sucks.

Layering isn't perfect, but it handles most of that. Even if ~80% of players quit, all servers should be pretty healthy. Not so many people quitting? It's a lot better to open up a little more servers (while still having problems). And lastly, from my understanding blizzard has stated that layers in the stress test are much smaller. Of course org is going to look empty when a layer has 300-500 people scattered across all of azeroth. Once layers have 2500+ (like it will at launch unless they are lying to us) cities will be much more populated.