I believe it's not just TSMC. I saw a presentation from one of the ARM architects a couple years back where she said that 28nm would likely stay the most econom

I believe it's not just TSMC. I saw a presentation from one of the ARM architects a couple years back where she said that 28nm would likely stay the most economic node for quite some time to come, as the nodes below that have increasingly expensive mask costs due to multi patterning, or get into very exotic light sources.