I also think decreasing the difficulty won't have the same effect because like what franky1 said it will speed things up and will ruin the countdown of the block reward halving. Let's say they reduced the difficulty, if the miners still follow the countdown regardless of the blocks mined they would obviously still mine more than 210,000.This would also lead miners to mine more empty blocks.  I think those could be the reasons why there's no fork for decreasing the difficulty of mining.