If bitcoin become legal across the globe that will be the start of the bitcoin mainstreaming and bitcoin will be used as a legal tender. But even with the legalities bitcoin needs to overcome some big issues such as slow confirmation rate, areas without electricity and internet connection, poor people who cannot even buy computer or smartphones. So possibly it bitcoin is legalize the usage of bitcoin will still be limited in urban areas with people who can afford to purchase computer and smartphones. The problem will hit the extreme poor people and the people in far flung areas that has no access to electricity and internet connection.
You are absolutely right. Certain rural areas will be affected greatly. Fiat won't disappear easily. It will take decades for bitcoin to be generally accepted everywhere and used by everyone. The unlearned ones would find it difficult to understand what it is, much more use it. We have a long way to long to go ☺️.