Now with the new cabin configuration editor we can finally understand how seat space works and what the problem with it was all along.
It seems that all 3 classes have the same demand for seat spacing, and that this is actually what was causing it to be extremely undesirable for economy class. It is also apparent that what counts is the total length of the seat (base seat length + extra inches length).
This means that to get a good rating for seat spacing you are almost forced to use recliner seats so it is never worth doing for economy class passengers.
I think this is a strange approach. Just like with FAs and other services economy class should have significantly lower demands so it's balanced. Right now you have to provide 10+ inches to get a noticeable effect on rating for economy class, which means removing a silly amount of your seats.
I disagree with this choice, IMO 4-6 inches extra space should be enough to give 5 bars rating on the bigger seats targeting economy class! 6 extra inches is so much room for economy that it would be considered absolute luxury (for the cheap ticket price).
For Business and First class the space rating works fine.