Coexistence is the only option, IMO. A few years ago, Sweden and some of the other countries wanted to replace physical cash completely with digital cash (fiat in both cases). But after a few years of compulsion, they just gave up that effort, as a lot of people simply refused to trust digital cash. Especially the elderly people find it very difficult to use their mobile phones to make payments. The same issue will arise with cryptocurrency as well. In fact, it is one of the reasons why we don't have a lot of senior citizens using cryptocurrency.
Exactly! The old generation is far from the adoption stages of digital era. Even if we consider the developing countries along with the underdeveloped one, BTC global adoption is next to impossible. It's being used everywhere, that's not a question but at what level it's being adopted is real thing. A global adoption would be something with the proper infra-structure. Ability to transact with crypto everywhere, irrespective of nation where I am paying the bills or holiday trips! Even today, digital world is lacking behind and everyone carries cash with them for emergencies!
You can stumble upon places where there is no network coverage area but local businesses exists and you want to buy something then how would you pay? Cash!
These are just small case studies, but can give you clear idea as to why digital or crypto based currencies lack the phenomenal growth.