*Changes to the rules that govern the algorithm would need to be done by the community in well decentralized and transparent manner.
There're two problems with such systems.
1. How does the system adjust rewards? One way is to ping exchanges for price volatility. Then how does the system choose the exchanges to include? The biggest 5, 10, or 20 by volume? Then x% of exchanges can collude and destroy other exchanges.
2. People can game the reward system. A dynamically adjusted reward system introduces a new variable. Mining can allow gaming because it's associated with real-world production costs. Your gaming is bounded by a physical reality. Gaming block rewards will result in financial manipulation. Let's say you own 10% of the coin supply, nobody comes close to your supply. You can start to manipulate the supply/demand to optimize your price. Your gaming is unbounded. Eventually you can control all supply, and therefore price.
I think a fixed rate would work better than those designs. It's not perfectly stable. But that's the tradeoff to maintain decentralization.