why would Ix would be 0?
It ought to be intuitive that Ix must be 0.  


Ix has no loop therefore cannot  be anything except 0 in this model.


"Current coming out of the 5V source"  only returns in a loop to 5V -ve side.


3V is fixed  so i must be 1mA
20i is a current amplifier. 
ix and i are not related since ix does not exist. (0)  
But in this model since i=1mA , Vth= 500mV