Yeah it sure seems that building more industries seems to be deleterious to profit margin. It seems that all it does is add prodcution and maintenance costs without adding to domestic sales.
Actually ur wrong and correct at the same time...u see, ur economy slowly adapts to its new enviroment. It can only do this about 3.5%+- gdp/c per year extra without increasing/decreasing unemployment and inflation.
So while u may start to PRODUCE more than 50% (wich should mean that ur people get 50% more pay and can spend 50% more, but that's not how it works) ur wages are still behind alot. So unless ur able of getting rid of excess goods that ur own pop don't use because they haven't reached that income level yet then it's best to just scrap those buildings...so always export while ur making a good surplus on it or just put it on demand.
If ur totally self-sufficient u can totally control how u want to work ur economy, ofcourse ur bounded to the growth inflation/unemployment rate %.
This is why the communist system in this game doesn't work properly. Somehow the communist plan system is using the same rules as the free-market economy of democratic regions in the game (how free-market that ever maybe will stay a discussion), in a communist nation u have a region wich produces X amount of resources and it redistributes them directly, aka ''production = income'' wich is only loosely followed in this game.