Output Resistance of Small-Signal MOSFET Circuit
You've written &quot;Current source whose current depends on voltage across it = resistor&quot;.
This is true, as long as the current is linearly proportional to the voltage across the current source.
However, the voltage across that current source is \$V_{ds}\$, while its current is proportional to \$V_{gs}\$. Thus, the current is not proportional to the voltage across the current source, and replacing it with a resistor is invalid.