The idea of all planets needing SOME infrastructure actually does sound like fun. Especially since pretty much all planets naturally produce infrastructure. As QuakeIV said, it will just soften that transition between colony cost 0 that needs 0 infrastructure for a population of 10 billion and colony cost 0.0001 (perhaps due to dust) that suddenly needs 1k infrastructure for that same amount.

Still, there will have to be some lower softcap and an upper softcap. A garden world with perfect conditions and 100k colonists doesn't need infrastructure. People can live in tents and pick the food from the trees around them. But if you try to cram 100 million colonists on that same planet, that becomes a slightly more tricky proposition and it makes sense that at least SOME TN tech will be necessary, if not crucially so for survival, at least for obtaining maximum efficiency.

Basically, initial colonization shouldn't need infrastructure but if you actually develop a colony it should.

Edit:

I spent some time and developed a proposal for the actual infrastructure needed formula. Currently, the formula is Pop (millions) x Colony Cost x 100. This could be seen as "100% of infrastructure needs are determined by colony cost". If instead we change that to "80% of infrastructure needs are determined by colony cost and 20% by maximum population" we can derive the following formula:

Infrastructure needed = Current population x Colony Cost x 80 + Current population x (Current population over lower cap / Max population over lower cap) x 20.

If your population is a very small percentage of the maximum capacity of the planet, the second term is going to be very close to zero. If your population is exactly equal to the capacity, 20% of your population is assumed to be living on colony cost 1 under the old system. If you have double your max population on a planet, each million population needs 40 infrastructure BEFORE considering the effects of colony costs.

Effectively, this results in the impact of colony cost becoming almost negligable at really high population densities. If you have 100x the capacity of the planet, each million population requires 80 x colony cost + 2000 infrastructure. The amount is absolutely silly high, but for really small bodies it is relevant as you might consider dumping 50k infrastructure on an asteroid to get to 30mil population to run your forward fleet base. This would represent building out and hollowing out the asteroid until there is very little of the original rock left.

If there is further interest in this, I suggest we split it out into its own seperate discussion.