I do agree that there is a possibility be insidious externalities, though I don't see them, but I don't see how it can work like having a sweet spot. In my proposal the rewards are just trash unidentified gear, and not very rewardingly. Any amount of time a community intentionally lets it lie inactive gives less rewards than doing it twice. The progressive rewards completing a quest would give you are hidden to save computational power and implementation effort. There can also be some degree of randomness in the progressive rewards.
Yes, players wouldn't know which stuff had an inactivity bonus, and which didn't. The point is to encourage diversity in the content each player plays, and building more community knowledge and activity in dead content, hoping for a lucky jackpot, that doesn't contain anything rare anyway.
Come to think of it, the progression of the progressive rewards can be personalized as well, and need not be something that can be reset by others. This is probably a better idea. In this other implementation, the rewards may be visible to the player. This would naturally give incentives to returning players. The progressive rewards should still not be very valuable, such that they should not displace farming the usual metas, but are eventually valuable enough that time and again one might run a grand tour of content.