I must say, it's not really flat (but I understand what you mean)... it depends on witnesses, which is the "I would say" most "customizable" method among humans that one can use on decentralised communities, to stabilise value among the stakers. But historically, it looks like flat (those are almost like a fact)... I agree!
Looking here... just for the sake of having a picture... Love pictures!
(visit https://hbdstats.com and check the graph from 
It looks great, the idea of mixing compounded interest with HP (Personally, I love the idea of the entire post), but in here, static would not work. In my view...
Unless the idea of scaling is also with a "ratio" decided by the witnesses. Unfortunately, the price of Hive is not dependent on just investors... but also attackers... and those, only witnesses can defend at the last level of opportunity in my view. Voting is required... indeed...
This one is the most interesting case in my view. As an enthusiastic (pool guy) interested in "staked" governed liquidity, I understand that it's crucial for investment and price stability.
But again, this one needs witnesses "stake"... otherwise it can be exploited too. For good or for bad, but ultimately, the survival of the chain needs to be the reason we build things on top of it.
But overall, if there was a variable that could min/max this influence from the witnesses, in order to drive/control the Debt of HBD (at least short term... because long term, VERY high volumes, HBD is possibly automatically controllable, the way it's meant to work).
So, for me, the addition of these would be quite a contestable scenario... I would love to see it tested. Unfortunately, I think that only using a realistic (HIVE) community, this could be really tested (Testnet will not help, as it depends on humans).
There are ways to test these, though... with a smaller scope... I am sort of interested in testing those kinds of things. We should talk on 
Note: this is my first impression in the middle of the night... I need to think more about it.
RE: An Open Idea: Linking HBD Interest Rates to Staked Hive Power