The only effect that can really drive a declining network hash rate is bitcoin price dropping at a rate that efficiency increases in hardware cannot meet.
Just imagine what might happen when the block rewards get cut in half...
Something's got to give.
We already saw one, and it's not necessarily a big deal because the moving parts are happening at different times.
At the exact moment a black halving happens, the machines that immediately get taken offline are those that are at a point in their life cycle where they weren't producing bitcoin revenue at 2x the rate of electricity consumption.
Depending on what kind of efficiency curve we see on waves of deployed hardware at or around that exact moment, the proportion of existing hashpower that falls below that efficiency threshold may not actually be all that significant.