In the past decade, optical line interface technology has been the key enabler for cost-effectively scaling optical transport networks (OTN). Increasingly higher data rates per line interface have been accompanied by higher spectral efficiencies, enabling to reduce the cost per bit transported and also mitigating or even avoiding the need to roll out or lease additional fibers. Nevertheless, as technology nears fundamental limits, the reduction in cost per bit transported granted by future line interfaces may not be followed by gains in spectral efficiency, highlighting the need to investigate alternative solutions to scale capacity. Considering a reference transport network and extrapolating how client traffic rates and line interface baud-rate will evolve, this work presents a preliminary assessment of the potential benefits and shortcomings of future generations of line interfaces.