Is this so certain, though? It seems fairly clear that Britannica wouldn't be using copyleft licensing, but proprietary licensing of some sort. Speaking for myself, I know I would not be contributing to a proprietary EB wiki. A lot of the initial seed for Wikipedia (for starting the virtuous cycle/exponential growth) seems to have been essentially ideologically motivated, and I do not think any significant segments of technically literate, motivated, educated and willing-to-donate-their-time people would have tossed their lot in with an EB wiki, except perhaps some academics.
The idea that a Wikipedian's work will change and adapt--will be immortal--is quite wonderful. The fact that they are codifying knowledge freely so that it can be reused is very satisfying.
I was made aware of the whole FLOSS movement because I became a Wikipedian (I did not become a Wikipedian because I was a member of the FLOSS community). Again, I expect this is the case with most users. I was initially attracted by the freedom to edit and the projects ability to grow (reuse of our work by others as a result of free licensing is an extension of this). I can't say whether I'd still be editing if this were a proprietary project, probably not (I now strongly believe in FLOSS principles and can't envisage giving my free time to something proprietary, so I may be biased). As for those who are less philosophical, I think there would be enough of them to have made a proprietary Wikipedia successful (in the absence of a free alternative, which would be more attractive to most).