Such a significant reduction in block confirmation time from 21 to 12.5 seconds became available after the implementation of Constantinople. This is a historical minimum for the time of confirmation of a block on the air network.
Completely get rid of the bomb complexity for the air can only be by switching to PoS. Constantinople - the second case of "delaying" the bomb complexity on the network of the ether. Previously, the complexity of the bomb was already postponed once - and then it was possible to achieve a reduction in the block confirmation time to 13.5 seconds, followed by a slight increase.
Recall that a bomb of complexity implies an exponential increase in the complexity of mining blocks up to the impossibility of mining. Constantinople again delayed this moment, while the air had not yet managed to switch to PoS. The next stage is the PoS Beacon update, which can be implemented by the end of 2019.
