I am aware that the lender will spend some money back into the economy which might end up in the borrower's bank account as profit. Assuming 20 mil BTC is the total amount of BTC in existence, in your example, the lender will have to buy 10 mil BTC worth of good and services from that country so that in the end, the lender gets 30 mil BTCs as 20 mil in coins + 10 mil in the form of good and services. Right?
Well, I'm not sure that is the perception that many people have when making a loan. If you give a loan for $1000 with 10% interest, you expect to get $1100 after a (long) period of time, not to get $1000 + a $100 voucher worth of goods and services. I'm not saying that we shouldn't go back to the "voucher" system when money are backed by good and services, maybe that would be one of the fixes ...
The real issue arises when the money loaned is magiced out of nowhere. If I have $100, loan you that, get $110 back, that's one thing. If I don't have $100 but magic it from nowhere, lend it to you, get $110 back, I have obtained $10 for nothing. That is what the government/federal reserve is up to at the moment and it's a problem.