If only the Witnesses got rewarded, and everything you proposed was implemented, the only demand for STEEM would come from people buying it to affect the board of witnesses.
STEEM will lose its attraction as shares of control over the reward pool.
I once saw a proposal to create an image of STEEM, a hard fork which will delete all preminers and their effects, which is impossible, because the vast majority of notable accounts was affected by preminers by delegation, votes, transfers, etc, so a deletion of preminers and their recursive effects meant deletion of all 3 accounts of the issuer of the proposal himself, as I noted then and deletion of all of the witnesses and most notable posters.
STEEM is based on sins and doomed to fail.
Every cryptocurrency is based on sins but I am not as sure about every cryptocurrency's destiny.
Most of them including bitcoin are either unsustainable, or uneconomical.
Spam and corrupt preminers have been failing this experiment since its get to.