Right or wrong, the 10 Gb/s and 40 Gb/s fiber links already existed for SONET, so their application to Ethernet was logical. Beyond 40 Gb/s is new territory.
I would expect that when the 100 Gb/s serial Ethernet link is developed, any idea of 50 Gb/s will become obsolete. Seems to me that the only reason to consider serial 50 Gb/s is that it should be easier and quicker to develop than the serial 100, and that's about it. Otherwise, what's the point?