Not sure bitcoin and income tax are in the same dimension and thus a realistic connection can be drawn between the two to make predictions: bitcoin is a type of value exchange mechanism, in other words it is an economic construct. Taxes, in turn, are primarily legal constructs, i.e. they are essentially rules enforceable in a specific country's court system. Bitcoins can be taxed or not, fiat currency can be taxed or not, any combination is possible and depends on the state of the specific legal system, imho.

In other words, here's a simple 2x2 matrix where any cell is a possible situation; the bottom left cell is the situation in most countries at the moment:

+------------------------------------------+--------------------------------------------+
| Taxes exist, bitcoin has legal status    | No income tax, bitcoin has legal status    |
+------------------------------------------+--------------------------------------------+
| Taxes exist, bitcoin has no legal status | No income tax, bitcoin has no legal status |
+------------------------------------------+--------------------------------------------+

Some examples: firstly, imagine a certain country decides to accept bitcoin as legal tender (i.e. equivalent to money), the necessary prerequisite for that to happen is that the banking system starts working with it on par with the national currency. If that happens, banks will do KYC and AML procedures on the incoming bitcoins, which essentially removes anonymity from the latter. And allows it to be taxed just like any other type of value exchange mechanism. Another example from the above 2x2 matrix is a country with no income tax (e.g. Estonia and Latvia don't charge corporate income tax unless you withdraw dividends) but no legal status for bitcoin or other cryptocurrencies. The remaining two cells of the matrix are also easily imaginable.