Why decrease rewards at all? Rewards exist to provide incentive to create value that inures to Steem. Decrease rewards, decrease the value of Steem.
SPS will fund development, and that development will be intended to increase the price of Steem, creating capital gains and increasing the value of stake. Funding for SPS should be drawn from stake directly, rather than decreasing incentive to create value in Steem, and this hit's all stakeholders equally, from redfish to whales. It also potentiates capital gains for all equally according to the stake that capital gains will impact.
Retention is a horror show presently, and decreasing author rewards by almost 10%... I don't even... Well, any decrease in author rewards will decrease retention more, and we are not growing now. Many witnesses are barely breaking even, or not at all, now. Witnesses will flee in droves if we drop their rewards almost 5%, leaving the blockchain much less secure. Decreasing witness rewards is the last thing we want to do.
I disagree that SPS should be funded by decreasing rewards at all, but what you propose is at least better than the scorched Earth 20% proposed above.
Let's just eliminate curation rewards completely. It's just rent seeking anyway, and has no relation to content quality. In fact there is an inverse relationship between increased curation rewards and increased content quality. The more profitable curation is, the more is delegated to bidbots, and the more curation is just ROI without any consideration for content quality.
Eliminate curation rewards completely, and the only reason people would vote would be to reward quality content.
I could live with that.