Happiness effects are miniscule compared to the CG costs. To give a real world example:
+416/+837/+1.3k resource rates on impoverished.
+670/-375/+1.3k after switching to just Social Welfare in my game.
So yeah, I stand by my assertion that CG costs will bankrupt you and especially your sectors
No need to repurpose buildings. -20% happiness penalties are bearable so long as you handle unrest. Slavery is just a nice bonus. The only way to make stuff useless is to let unrest stay high.
....
At a glance, not sure your numbers reflect what we're discussing (are your numbers reflective of a net -30% change in happiness since impoverished to social welfare is net 20% change?)
The non-slaving scenarios seems to be:
(a) utopian abundance + recently conquered for net -5% happiness 10 years, then impoverished conditions forever
(b) impoverished conditions + recently conquered for net -35% for 10 years, then impoverished conditions forever
I'd be interested in seeing a more thorough analysis since the happiness production effects of utopian abundance to impoverished (as you suggest) is a net -30% happiness, whose effect on production varies highly based on what faction the pop is in and how happy that faction is. Especially with the happiness production deadzone of 40 to 60%, which a net 20% happiness as you tested could have totally not shown up much (if most of the pops were in a faction at 50% happiness, social welfare would give them 60%, and impoverished conditions 40%, making no change in production). And a truely thoroughly analysis would consider all the resources - minerals, energy, food, and science, but it'd probably be hard to do.