Why can't you just reduce the current instead of the voltage?
Your approach (10V and 2 Ohm series resistor) works (only) for a circuit that draws a constant current of 2.5A at 5V. 

When the circuit draws more or less current it will see a voltage different from 5V. For instance, the circuit could behave like an 8Ohm resistor. The total resistance would be 10Ohm (they are in series), so the current would be 1A. The circuit would get 8V (1A * 8 Ohm). That will probably cause it to malfunction, or even damage it.