Time preference and the general "risk-free" rate of interest are just the aggregate effects of these sorts of opportunity-cost considerations being made by all the holders of currency in the market. Most of the time the ROI is more subjective than the fixed 5% return in the example, but it is no less real for that. No one is taking advantage of anyone else; lending at interest is nothing more or less than redistributing ready capital and future income to where they are each most valuable, which is fundamentally the basis for any economic activity.
Your arguing that interest derives from Productivity of capital goods, so you and myrkul need to have a debate on what the true source is interest is because your presenting an incompatible argument to his. I find your productivity argument weak because the Return on Investment of capital goods can and dose become negative when their is an excess of capitol, something that frequently occurs when entrepreneurs pile on to a profitable sector of the economy and saturate it. Eventually enough unprofitable businesses go bankrupt to restore the returns to at least a break-even level.
With sufficient competition the return on all capital goods should fall to zero and stay their, to what ever degree that ROI is above zero must derive from something that prevents it from dropping. And that is clearly money interest, once the capitol ROI is driven down too or below that of money then lending for additional business expansion in that sector stops. Thus the rate of money interest acts as a 'floor' to ROI of capitol goods. Your explanation is thus backwards, money interest creates capitol ROI not the other way around.