I can't really see any cryptocurrency being an efficient source of income. The price fluctuates way too much. One day you could have 50,000$ in a certain coin and the next day you're flat broke and can't pay bills. Of course it would work if you were very, very good at predicting the price of the coin so you can exchange when you feel the price way going down, or get more Bitcoin when you think it is going to go up, but you'd already most likely been in luck for a stable job if you were good enough at that. 
Suppose if you saying that in an inefficient source of income ? Let me give an example if you are interesting in these coins and want to take risk to become a millionaire. So you would go for bitcoin.
Because bitcoin has changed the history. Bitcoin has the changed the life of people. If those who has invested into other altcoins which is turn into scam so it's their fault to trust those altcoins. Bitcoin is trusted and it has gained a lot of people in a few years.
If it was such a risky or a scam, people at such large amount wouldn't dare to invest all of their property or money.