The game is not dying and hasn't been for at least a decade.
What's the difference between leveling up on mushrooms vs leveling up wherever you are now?
Maple has always had you move onto the next area for leveling up as soon as you were done with the old one.
There are plenty of optional reasons to visit old areas.
Monster Collection, Quests/Lore, Achievements, Familiar cards, etc.
If you make the old maps mandatory then people will get sick and tired of them as well.
If you're suggesting to add more optional reasons to visit these areas such as adding more achievements, that's an idea I can get behind.
The reason cannot be something that has real incentive for all players however, otherwise it becomes mandatory and you create the same problem.