Good point, but without being forced to vote, how can you guarantee the overlap even with infinite witness votes.
The second biggest problem besides "not enough overlap" was that most people did not use all their votes or had not voted at all. Especially some of the smaller accounts just learned recently that it is important to vote for witnesses. Ask anybody of those who didn't know that voting is important if they know what a voting proxy is and you will see that most of them have never heard about it.