Level Cap

Yeah and I think a lot of good old school D&D modules were actually setup that way. For example Dragon Magazine #50, Chapel of Silence. The PCs are level 2-3 and find a ring of wishes in the 2nd room of the dungeon. Later on they'll be facing a cockatrice(save vs petri or turned to stone) and a powerful vampire. They're supposed to use the ring to help with those encounters.

 

In many cases, killing monsters may not be your goal.  Bypassing them, bribing them, trapping them, locking them out, hiring them, sleeping with them, tricking them, scaring them off, or abandoning them may serve your purpose just as well.

The same goes for the monster's goal! It's not that monsters are here just to slay adventurers. Each monster has its own agenda (aka: Instict).