Any loss of contributors is a loss, and would demotivate the remaining contributors.
Nupedia wasn't at OSM's stage of development when Wikipedia forked off from it.
What I think is wild and bold is the proposition of a fork when there's no current need for it. If the licence debate gets to a point where contributors start leaving, then yeh, I hope those contributors at least work on another free mapping project. Some things are worth forking over. But for so long as everyone can continue to work together, it's probably best to do so.
If forking was so harmless, why only fork in two? Why not invite OSM to fork into ten little projects? :-)