I think you make a compelling argument. Upvoted and followed, I'll be watching closely to see how this idea develops.
Unquestionably more marketing resources is more useful than the 1.5% SP interest. The question is, why hold Steem Power then? There's something missing in our current incentive structure there. The problem isn't keeping your money there; Voting and security benefits are nice, but only emotionally apparent after you've powered up. The trick is how to get newer members to Power up.
That's the incentive to ponder.
Yes, however little incentive there currently exists to hold SP, the 1.5% yearly interest which is about 1/4 of the daily price volatility, is adding next to nothing. If the price creeps up another cent, it'll be at the cost of $1m a year for no tangible reward.
There are a lot of discussion about whether we even need SP or even SBD at all, but this is a separate issue and can be changed (or not) in due time. I'm currently only focused on this 15% as I think the $800,000+ a year shouldn't be wasted any longer than necessary.
With regards to the question of why hold voting power at all? my personal belief is that this is an incentive that has to arise organically from the desire and willingness to partake in a robust platform or ecosystem of platforms. The bigger and better Steemit, Squeak, Steepshot, Busy, etc are, the more people will naturally be incentivized to take part and have a voice by building upu voting power. There are no short cuts to this.
Any direct attempt to bribe people to hold voting power, be it in the form of higher interest rates, or dividend payments, etc will come at a cost to the ecosystem, such as higher inflation or transaction costs, that would likely come at an overall net loss due to market friction and imperfections and the manner of wealth redistribution in question.
Ultimately Steem can flourish if it is able to redistributing wealth in a psychologically frictionless manner towards quality content. No further coercive incentive is necessary or desirable. Building engaging platforms that hold quality content over time is the only way.
There are economic nuances here that can be argued here of course, and I may be wrong in some sense. But my instinct here is that's how it should be. I wonder if you are persuaded, share your thoughts with me if you have the time
PowerUpCoinDays, which could be used to promote content on demand
You increase the curation rewards. Give people a reason to want to reach large SP levels.