Cash has nothing to do with banks. Banks do loans or better banks link those that need money and those that have it on store. More then half worlds population dont have this option.
Cash is fine if you deal close by. If you deal over some distance people mainly use some banking solution payments and that takes time.
No, the Bitcoin is digital. It can only secure digital asset and they have most problems with copies. Bitcoin will not solve counterfeit of food, shelter, clean water, and clothes.
So "the GDP will grow a lot," as you said because transactions occur in a long-distance, a.k.a online payments? And it can secure digital assets, hence removing the counterfeit digital assets?
What if there's no (or only a minuscule) digital asset to be secured in the first place if you talk about developing countries. Plus there are fintech companies offering pretty much instant transactions with QR code and even phone number?
I definitely did not focus on digital asset in developing countries. This dont lead anywhere if you believe there is no no digital asset to be secured. We seems to live in different worlds.