The Fibre Channel argument is a false one. Implementing NVMeoF is only possible on the latest releases of Fibre Channel (16/32 from memory), so customers will have to replace technology somewhere. I doubt end users are fully 16 or 32Gb/s today. So if they have to rip and replace, it may be the perfect time to look at harmonising networks and using Ethernet.
What's more likely is FC gets retained because the storage teams don't want the network team messing about with network topologies they don't understand. Plus, isolation sometimes is no bad thing, if you have the scale to justify it,