I feel my wording was not good.
With reducing the difficulty, we don't really decrease the energy consumption. Because all miners are still working and the total hash rate of the network wouldn't change. For decreasing the energy consumption, we need more efficient miners.

To change my previous statement and to be more accurate:
For decreasing the energy needed for mining a block, one way is to decrease the difficulty and the problem is that it can lead to a 51% attack.

OP suggested to make the amount of power used to mine blocks lower, so miners are encouraged to mine blocks with low transaction fees.
To do so, we would need a hard fork decreasing the difficulty. I know how this can make the network insecure and I'm not saying that's a good solution.
 
Making more efficient miners is another solution. But that's responsibility of ASIC manufacturers, not bitcoin developers.