A blockchain is only as secure as the rewards given to the block producers to secure it. If the reward is not high enough, quality people go elsewhere and the security of the network drops. I'm not convinced increasing the number solves any problems because the amount available to reward them is the same (unless you increase that amount which would probably take away rewards from authors and curators) so each witness would earn less. The problem was a sybil attack where one stake holder can convince exchanges via lies to control the whole chain with sock puppets. It wasn't about the number of individuals because one individual controlled all of them.