Sharing the Load: Power Ratings
The most likely flaw in your reasoning is probably that you think a potentiometer used as a rheostat can dissipate it's rated power at any setting of the wiper. It can't.



Figure 1. The potentiometer resistance track. Source: Types of resistors.

The rated power is when the dissipation is spread over the full length of the resistance track. When used as a variable resistor (rheostat), as you are proposing, the maximum power dissipation is reduced in proportion to the amount of track in use. Effectively the power specification is telling you the maximum current the track can handle.

Let's say that the potentiometer in Figure 1 has a rated power of 0.125 W and a resistance of 250 &Omega;.

The maximum current it can handle is (from \$ P = I^2R \$) given by \$ I = \sqrt{\frac {P}{R}} = \sqrt{\frac {0.125}{250}} = 22\ \text{mA} \$.

At, say, a 40% setting the maximum power allowed would be \$ 0.125 \times 0.4 = 0.05 \ \text W \$.