The profitability of hoarding bitcoins can be compared to interest rate...
No it absolutely can't! Interest rate is a tool central banks use to give the currency a value because the potential total moneysupply is infinite, and thus intrinsic future value of a USD is ±0. Now that can be considered a ponzi, because the people who get to spend/borrow the new money FIRST get most for their buck while the last people in the business-cycle pay inflated prices. (last pay for the first)
But in bitcoin there is no interest rate. Because bitcoins are in themself attractive due to a fixed, predictible supply similar to gold, nobody gets any benefit of spending/borrowing the money first or last.
Hoarding, is not an investment, its a speculation. And it can lead to losses.