A few ideas, some of which are silly. Its hard to stay true to the original game while trying to solve problem of sustainably caused by unlimited levels and invincible items.

- Increase food difficulty to obtain, duration and price. This will increase the running cost of all players. Low level players could be incentivised to spend time gathering food and selling to high level players, similar to the rune system.
- Reduce magic scaling with level. Controversial but this feature makes hunting in the hardest areas more and more profitable decreasing the need for high loot values to sustain hunting.
- Slightly reduce (80% maybe) income of minotaurs/ dwarves.
- Slightly increase rarity of key items like giant sword/ dragon lance, etc.
- Reduce low effort rune production. I am not sure what the sleeping runemaker situation is at the moment.
- Exp or skill decay, or increase of blessing cost, or consequence of death at higher levels such that players plateu.
- Have an age system where your character gets old (binary young or old) and somehow made weaker so that the most powerful players are in the young category provided they gained enough levels and skills by that point. This would encourage players to remake characters. Similar to a ladder season on other games.
- Item endurance or breaking. I don’t like this personally but having items that need intermittent repair could help increase the cost of hunting, or even make players consider whether to use a weaker but cheaper to maintain item.

If in doubt ask the question - “what stops this happening in real life?”. For me the answer looks something like: people die, stuff wears out, population increases constrained by limited resources.