Edit: Thus... my question
here.If your question is "do full blocks, and a transaction backlog increase the security of Bitcoin?" the answer is clearly yes, as this would induce bigger transaction fees, thereby inducing a bigger incentive for miners to secure the network.
Maybe it's possible to find a metric taking in account average backlog to grow the block size. (but one that can't be easily or too much gamed, not sure it's reliable)