I don't object to increasing the rewards pool, and maybe witness pay. Your point that the interest rate is negligible in comparison to price volatility is valid. But as to the development fund, isn't that the purpose of reward splitting that was introduced in HF18?
Arbitrary Reward Splitting: Yes, reward splitting is here. Now you can monetize any app you build on Steem!
Unless I'm mistaken, Steemit, Inc can also make use of this capability if they find it useful. They don't need their own separate slush fund, and I doubt if they'd agree to accept it when the reward splitting capability exists.
My gut feeling is that it should just all go into the rewards pool.
Well that's still certainly better than how it is right now
I suppose I would prefer it if Steemit got funding consensus from the community, rather than just unilaterally pulled a tax on rewards from a certain point on. My guess is their next project outside of steemit would try to make use of this feature, but would like it if Steemit Inc made platforms without seizing rewards directly, but we as the entire community reward them with a slice of the overall pie. The alternatives are they start charging a 'tax' on steemit one day which doesn't need our consent, or they start working on other projects and set arbitrary rewards, which could be at the cost of maintaining this less rewarding Steemit platform.
But I definitely prefer any use of the 15%, including throwing it all into rewards, or reducing the inflation by that amount entirely, over what we currently have.