Please also see that Poet's OI is not limited to 400G. It is a platform technology which will mature over time. The physical limitations are - at the moment - to me not visible. But I expect that 1,6 T etc will follow if and when needed.
Further potential will evolve. Going to 400g and beyond the bottleneck of data communication will move from "between server racks" to inside the racks, then inside the chips. Poets OI platform can be applied at chip-level - to help the communication between "chips" and inside e.g. a CPU, - for the OI all this will make not much difference. So once the principal of the OI is approved, further potential is - nice.