True enough about typically social media post not generating continued interest, which is why I think this could work without bogging down the system. It would benefit the few cases where someone shares content that does create renewed interest like, a tutorial, or a music video, or artwork, etc. It would shift the incentives here to quality over quantity.
Also love the resteem idea, I think it would be cool if the poster could set the amount with like a minimum of 15% or something. But imagine you're promoting something, you might set it to 100% if your primary goal is to spread the word about something.
I like that aspect of it being a customizable field set by the author. You’re absolutely right it could be a strategy to get more exposure and promotion.
After thinking more about my reply, I thought of a troublesome exploit though, someone would post something on account A, have 100 alt accounts that all resteem the post and have account A set the beneficiary amount to where the resteemers get 100% of the payout.