Notwithstanding the separate debate about using bid bots. If a post is not worth promoting after 3.5 days then how much does it add to steem/the world? That is one of the issues of the 7 day payout rule. It reduces the incentive for material that lasts.

WRT promotion itself however - my understanding was that votes were intended to allow humans to weed out the good material where bots don't do that. They promote the material of people who can and want to pay for it.