You're basically wrong, I'm afraid.
The 737's on the route were 500's - with 113 seats. That's a major difference from 152 on a A320. 320's and 321's would currently be used as backup aircraft; I would be most surprised if capacity is actually being sold to 320 levels, let alone 321.
As for loads, when they went from 5 to 3 flights a day (a reduction in capacity of 40%), the reduction in traffic was (I am led to believe) less than 15% - you could probably get these figures from the CAA. So you can imagine what that did to the income per flight. With the current low cost fare structure, the income per flight will be substantially better still - since the more full a flight is, the higher the average fare.
If the airline was making loads of money on the route (as it would have been if the flight was "fairly full every time") do you think it would have wanted to use the slots for something else?!