Civilians using TN materials would eliminate the primary reason for having them. They would simply be too expensive to ever allow.
I don't think that needs to be the case as it would be a different mechanic. i would not be so quick to judge something before there is a general idea how it would work.
In my opinion all civilian population should consume some TN materials all the time, it they don't get enough there can be some wealth issues. I'm more interested in the simulation of the civilian part of the economy and how that can force problems and decisions on the players general expansion and strategy.
In my opinion obstacles usually leads to interesting choices rather than just progressive advancement. That is why I think that allowing civilian population and civilian freighter lines to consume resources is a good idea.
It obviously would need to be balanced with other incomes and also how wealth in general work. But I think that reverting back to the population generating wealth if it also require resources would be better than the current situation. We also should be able to influence to some degree how much workforce we want to use out of the population. The more of the population we use for governmental production the less wealth we will generate. We also could eliminate the financial buildings entirely as well and have the civilian population generate all the wealth.
You can even role-play a hive mind that way, the civilians just are the general drone population that generate the wealth as wealth is not really a defined resource.
You then would see populations service industry to be the one that generate wealth and the more population you use for construction the less wealth that particular colony will generate. If you use too much of the service industry you will eventually get morale problems. So you would still be limited to how many people you can use for governmental industry it just would be more or a soft cap.
Civilian shipping lines would be a very important cog in the trade between colonies. If trade is not performed you would get negative wealth income from those worlds... perhaps a world could then suffer from wealth income between say 5-100% depending on how much of its trade with other colonies it can satisfy. This would also "fix" conventional starts as the wealth income on the home planet are pretty limited at the start with a very low efficiency, although the home planet of any race could have a higher lowest cap of efficiency. The efficiency would also effect the consume of resources for the civilians as well so as when wealth efficiency rise so does the colonies need for resources.
The game need "in my opinion" more sources like MSP, fuel and the like drainage to make planning giving you even more decisions on what to focus on and how.
You now could not ignore the civilians as you need them. Although I also think that under such a system the player should have a governmental civilian company where you can add ships that you construct yourself if you don't want civilians to spawn by themselves.