Your belief that there is a problem is based on the assumption that 100% the BTC is hoarded and nobody spends it. Then there would be no ability to pay interest because there would be no way to acquire more BTC. That might happen in a hyper-deflation scenario, but it wouldn't happen normally.

These examples should prove that there is no problem.
On a small scale, I can loan you 2 BTC, and you use it to earn 4 BTC, and pay me back 3 BTC.
On a large scale, I could loan a company 2 million BTC, it manufactures products and makes 4 million BTC, and it pays me back 3 million BTC.
On a larger scale, I could loan a country 20 million BTC (over time), it uses it to fund Social Security, and it pays me back 30 million BTC (over time). Of course, in this case both parties would have to spend their BTC, or there would be a problem.