My experience is that specs - even will written ones - are far enough from working, tested code that it's going to be significantly easier to take the BSD project proprietary than to build a competitor to a GPL licensed one from scratch. Also, to be attractive to the 'core group', I think working on new stuff is more interesting.
Well, look, sure, if you hire all the developers that are working on something and understand it, you can probably out-compete other implementations even if all you have the old developers do on your proprietary project is write up specs from which a different set of developers build a legally non-derivative interoperable implementation.
That may be a risk with permissively-licensed Free software, but its also a risk with copyleft Free software and proprietary software.