Go to time 16:00 in this clip and think about how the demand for mux/demux will be filled. There are a lot of unknowns for 400G in terms of the number of lanes. Limited by the availability of lasers and temperature constraints associated with lasers that will be capable of 100G. So 16, 8 or 4.
https://www.youtube.com/watch?v=wwbQt6tmVgk