Sure, but if 6 million BTC's or 21 million BTC's are to lent out where are the new BTC's going to come from to pay for the interest. Gold became unsutable for modern finance because of this scarsity, so fiat currency was created. how can an economy grow?
I don't think we could ever reach loans as big as 6M BTC, or ever have all the BTC out for loans. That's like a loan for a bazillion US dollars... Of course it wouldn't get paid. Fiat currency was created so states could temporarily
create money to pay for sudden costs like wars, not to facilitate loans. IIRC, one of the highest growth periods in US history was while the dollar was pegged to the price of gold, before Richard Nixon reinstituted "normal" fiat printing to pay for the Vietnam war.