Anyways, to really understand what happens when R->0 I think we need to make a new model that takes into account what we just learned from your chart above (that miners won't necessarily be hashing all the time).
That seems to be an interesting point illustrating how the best interest of users and miners incentives could diverge.
For users, an empty block is always better than no block because it adds work to the chain and increases the security of the previous transactions.