The blog post for that video explains that as more people utilize their voting power then the ROI will decline (to as low as the Steem inflation rate which is 9% per annum and decreasing by 0.5% per annum).

It appears to me that the current ROI is about 0.33% per 5 days, and that is 0.58 – 0.67% factoring in that SBD are currently are $3.50 – $4.