As Nagalim and mhps have mentioned, while the block reward does remain constant at 40 NSR regardless of changes in difficulty, the average amount of time it takes to mint a block with 10,000 NSR changes proportionately with the difficulty. If the difficulty drops in half, the average amount of time it takes to mint a block will also drop in half, to a minimum of seven days.
This seven day minimum means that the network needs a minimum of 100 million NuShares minting in order to produce blocks at one minute intervals. My quick estimate is that there are currently slightly less than 200 million NuShares minting. The network produces 21 million NuShares from minting per year, so the current protocol permits constant minters to make up to a 21% return on the their NuShares in NuShares if the difficulty fell far enough (to about half of what it is now). At the current difficulty level, minters are getting approximately 12% annual return on their NuShares.
The need for 100 million NuShares to mint in order to keep block intervals at 1 minute should be understood by the technical people in the community because if the network is successful, it is likely that so many NuShares will be burned that the minting quantity will fall below 100 million. There are a couple remedies to this that involve protocol changes. One would be to reduce the quantity of NuShare satoshis required to mint a block. Right now it is 100 million satoshis, so this could be changed to 50 million satoshis, for instance. Another method would be to reduce the 7 day age requirement for minting eligibility.
I hope such a protocol change becomes necessary, but it certainly shouldn't be a priority right now.
A motion has been passed to reduce the per block minting to 20 if the NuShare supply drops below 500 million, although this has not been placed in the code yet. With NSR burns being paired with NBT sales and reductions in mint rewards paired with drops in NuShare supply, we can expect NuShares to become quite scarce if there is healthy growth in the quantity of NuBits.