Network difficulty is not directly associated with reward halving, in fact the hashrate should decrease in theory.
That's actually not good at all, it means we are looking at a significantly flattened (as in the curves are not as extreme) Bell curve for Bitcoin global hashrate between 2009 and 2140. I guess this is why people have been saying that more incentives for Bitcoin miners are required to guarantee that the hashrate stays more or less stable once block rewards in BTC denominations start to become scarce.