I think trading & revenue share business is the best earning source for bitcoin.Crypto currency trading is always involved high risk so well before doing research then doing trading will be done.
Bitcoin is illegal for few countries Because the government thinks that bitcoin will do much harm to their economy.It's someone uses it illegally in this county then the government takes legal action.
Actually it will be very difficult to give a future message about what will happen after twenty years.But Bitcoin is day by day most popular currency in the world.I think the future will be more popular it's my personal opinion.
BTC is digital currency or cryptocurrency or digital cash it's creator is "Satoshi Nakamoto". The question weather btc is a currency or not is still disputed.
There are many business for you using BTC but BTC price is not stable.Every minutes BTC price is up & down.You can try trading using BTC but trade is always risk lose will be a lot of losses and profit if there is a lot of profits.
Monthly fixed income is not possible from BTC trading.Because BTC price is always up & down,if BTC price is increasing day by day.Yet it is not possible to expect monthly income.