We all agree on the need for passive income but 'investors" will probably not choose to buy Steem Power for 6-7% considering opportunity cost and risks of crypto. If this were double, or around 12%, then yes this would attract investors but again why not focus on the SMTs? If SMTs can provide passive income, or higher interest etc, then investors can buy those and it can indirectly benefit Steem.
My opinion on the sweet spot is just my own opinion. Because crypto is so risky as it is, and 6-7% is barely above inflation, it's just not attractive when people can get the same from stocks with far lower risk and more diversification. SMTs can offer diversification along with the same sort of interest percentages and this could spread risk which would make it more attractive to investors long term.
The main problem I think right now is that Steem only has a few slots to earn rewards and the pie isn't growing. Not everyone wants to be a blogger and even people who blog eventually get burned out, or desire to contribute value in some other way. SMT provides other ways to contribute value which is the only way to grow the pie, but then investors need a reason to want SMTs, and investors need a reason to lock up (supply sinks).