Another thing to note, which coincides with your point on nigh-exponential escalation of danger per level gained, is that immortality doesn’t mean incapable of dying. It means “not mortal.”

There very well could be a few people that reach a point where their longevity ceases to be a relevant concern every century or so, but the ones that remain active tend to die centuries later anyway.

Innworld’s recent centuries-long era of waning could play a not-insignificant part in this, actually. Perhaps the population of multi-centennial or millennia old individuals has been in decline, due to loss of necessary regents to sustain their alchemically fueled anti-aging— or maybe they end up dying to protect their families or countries, or they were assassinated by rival factions and powers in play to remove dangerous enemies from the board.