The question is If bitcoin is the only currency in the world (as most of you expect), How the Economy will grow if the interest rate is not stable and is very high?

If the economy is growing, one bitcoin has bigger buying power, then why do not the bitcoin owner hoard the money rather than send it to the bank? You can say the bank will pay for the interest. So the lending cost will be the interest + economic growth rate.

How can economy grow? By investing and developing new things/methods to satisfy human needs. No lending, no investing and as a result, no economic growth (even there is economic growth, this kinds of growth will centralize bitcoin to big monopoly enterprises). The ROI(return on investment) must be bigger than the lending cost. Which means the ROIs must exceed the economic growth rate.

Let's assume there is only one project for the whole world with an ROI 10%. Then if the project can be done by its own money, the ROI of the world will be 10% and so the economy growth will be 10%. So where is the interest?

If there are two projects for the whole world with the same size and ROI 15% 5% each. then the economy growth rate will be 10%. People will expect the 5% project can not pay back the interest and will stop investing in it. as a result, the economy growth rate will be 15%/2=7.5%.

If people are expecting economic growth too high, the required ROI will be too high and few projects can achieve that, which will lead to a frozen in the lending market. A frozen market will lead to the withdraw of industry and is harmful to the society.

So here is the conclusion, fixed money amount is not a good choice.