The difficulty adjusts yes, but the total hashrate has to increase forever. It can decrease for a while but long term it has to increase or the risk for double

The difficulty adjusts yes, but the total hashrate has to increase forever. It can decrease for a while but long term it has to increase or the risk for double spend increases too. The difficultly adjusting does not prevent the network form attacks at all. If 80% of miners go out of business someone can buy the hardware from them wait for the 80% difficulty adjustment and then start the attack. He could mine blocks faster than anyone else so even if someone mines a block his chain will eventually be longer. There is no know way to recover from something like that.