We all knowe howe empty and dead most town have bean in old games, ex in vanila wow everyone was hanging in IF/ogri when it was the plase for the AH and the oter 2 towns was dead.

What can be done to prevent that?

What do you wana have to give you a reason to go back to old areas?

In ESO, One can re visit a cave or dungeon and it will tale a different story. Thus some high levels will come back to the old areas for that reason.

Well, one thing might be some daily quests that requires you to travel back to the old areas.

Master dungeons as well I believe, although that won't really populate the towns that much.

In a game with instances, it's easy to send players back to "old" areas that they have already experienced, because that door that USED to lead to the home of an old soldier who told you about his lost dog and sent you on your level 5 missions ~now~ opens into a home with a secret basemant door to a necromancer cult's base of operations that only your level 50 bad-ass self can clear out.

Without instances, though, I'm not sure what they can do to draw people back to lower-level areas. Any challenge to the higher level player-character would be an obnoxiously overpowered insta-gib threat to the people who were just leveling up for the first time in that area.

They can draw people back to SOME areas by having markets, banks, etc and NPCs that you just TALK to go get missions that then require you to go to level-appropriate areas, but lower level areas that are "hunting grounds" WILL be occupied only by newer characters and/or alts (who may not be there long, having already experienced the content once or more).

With the single server technology there should always be a healthy population everywhere