Even if you don't invest anywhere else, you are already invested in bitcoins.....
This is true. If your daily income is in a fiat currency, then you have already made your investment by buying BTc.
Presumably you have bought Bitcoin for a mixture of reasons
1) To pay for stuff with
2) To profit on through price increase
3) Because you think it's ideologically cool.
That's fair enough, but BTc is the investment, not the investment currency. Fiat is the currency, BTc is the commodity you buy with it. It may be a commodity which has financial applications, but it is the investment/speculation in itself.
There is a parallel with gold here. People buy gold with fiat because they want their "wealth" to be diversified out of paper. They don't then get their ingot or Double Eagle or whatever and think, right, what can I
do with this?
The vast majority of "investments" offered for Bitcoin are scams designed to appeal to the naivety and greed of holders who think they should be doing something more than just holding their Bitcoin.
Nobody actually
needs Bitcoin, unless they want to buy something which is only available or preferentially denominated in it and, apart from SR dr*gs, not many things fit that bill.
Fiat interest rates in most of the world are very low, so why should Bitcoin magically be able to return 1%+ per day?