Hash rate dropping is not itself centralizing. It's a natural and expected response to price speculation. It's not damaging to Bitcoin. Bulls just don't like to imagine a world where hash rate and price aren't constantly rising.
Although natural and logical hash rate dropping is dangerous, not damaging but dangerous.
An attack on the chain is complicated right now because of the price of the gear and most important the availability. You have to bribe lots of miners and while paying them more to get the hashrate by magic make sure the short time demand doesn't push the price increasingly higher.
But, with less profitability miners turn their machines off by themselves, dropping the costs for the attack but far, more importantly, creating a large supply of available gear that can be rented or even bought for pennies.
For example, right now we're at 90 exa, you would need close to two million s17 to match this if you can't rent anything, 4 damn billions. Even if you had the money no chipmaker will be able to match the order in a month or two.
What if the price drops to 3000 and we're back at the panic level that would put everything not matching 40j/th on hold?
Almost all the gear that was mining last October (around 40exa) would be turned off this time for sure and just increasing the number of miners that can be bought at scrap metal prices.
From billions, the whole attack will be ready for a few hundred millions...