> (ie: if I borrow 100 BTC from you today, I only will have to repay you 75 BTC two months from now

Interest is almost* always non-negative. I'll only lend you those 100 BTC if you pay me something more than I'll have if I didn't lend them.

That makes a BTC based economy more prone to hoarding, and less prone to investing (what is clearly a bad thing), and also more prone to hoarding and less prone to spending (what economists think is a bad thing - I have no idea why).

* The exceptions to this rule are not systemic, normaly appearing in times of disruption. You can't base an economy on them.

Why would I hold BTC in a bank that charges me for that service? I can hold them with complete safety in my wallet. The reason interest rates will rise is that there is essentially a base interest rate paid to people who hoard BTC due to deflation. In order to attract investors, I would have to offer an interest rate that exceeds the risk-adjusted return on capital for simple hoarding. Since that risk is effectively 0, and the deflation rate may be high, the interest rate I offer would need to be very high.

Indeed. But inside a deflationary economy, it is impossible for banks to make money off of deposits by lending them to someone else. So I dare say that banks cannot remain solvent with BTCs, the economics don't make sense.

That said, plenty of people are "banking" with online BTC wallets. Instawallet, Coinbase, etc. etc. The economics for these services work against them, and they are hopelessly doomed to failure.