you should Keep in mind that PoS was designed to secure the blockchain and confirm Transaction
as some Kind of energy efficience method ... compared to Mining via PoW.
and not for generating coins or giving x% interest per year as Marketing idea.
You are right and this is a big problem with PoS-coins:
- If the interest rate is too low, people will go to some other coin, where it is higher.
- If it's too high, the coin will die at some point.
Finding the sweet spot seems like an almost impossible task. Here's a crazy idea: what would happen, if users could vote on interest rate regularly, let's say, quarterly? You take all numbers proposed and find the median. Some will state crazy high rates, others zero, hopefully, a lot of people would be sensible enough to realize that choosing the right amount secures the future of their coin, so maybe, it would be a way to apply "collective wisdom" (I'm not necessarily a big fan of that concept in general) to this problem.
I think users are too stupid to make good decision. Having votes is ok but people are very bad at making decisions that serve their long term interest (look at election!)