Why are you stuck on interest rates?
Because real interest rates determine how successful a company needs to be in relation to the principal that funded it.
Until a company pays off its creditors, it has not been proven successful. A company that beats the inflation rate is successful in nominal terms, but has not necessarily profited in real terms (increase purchasing power).
So if the lender knows the inflation rate, he should (and will if he can) ask the real interest rate plus the inflation rate.
With deflation, he's not going to ask the the real interest rate less the deflation rate, he will get at least the lower of the two. If real capital yields go below the deflation rate, no lending at all would happen. Nothing. Lenders would just prefer to keep earning from deflation rather than investing in any real capital,
even if it is profitable in real terms.
In this case there's no real investment out there that outperforms hoarding, so rational investors should not lend.
But when the deflation rate is lower than the capital yields, the problem is still there:
Some business that are profitable in real terms will turn out to be insolvent in a deflationary scenario, because financial costs are in nominal terms and your income and collateral are being devalued nominally.
Inflation creates the incentive to invest in companies that are not profitable in real terms because doing so is better than doing nothing. While this creates jobs in the near term, it is detrimental to real growth. Inflation is a major contributor to the business cycle - bubbles and contraction.
You're talking about keynesian inflation, I just want to talk about deflation. I'm not advocating inflation. I'm just pointing out the problems with deflation.
I also disagree with the way you relate all the problems we have today with inflation. A bitcoin-like currency with an exponentially growing base but with miners receiving the inflation instead of commercial banks and governments in form of debt would have very different effects on its economy.
But again, I'm not advocating inflation, just warning about deflation.