Imagine, 20 million BTC are held in a bank offering 1% interest. They stay sitting for 6 years with compound interest, see the issue?

The reason interest had to exist in the first place, was to incentivize actually putting money into the banks, now the only reason to do it would be security. Additionally, if inflation didn't exist, I think people could learn to be happy with a 0% ROI in banks. With Bitcoin, it may even be reasonable for banks to charge anti-interest to accommodate the deflation that will arise. So long as the interest matches inflation, the system is fair, and you never lose purchasing power parity from your savings.