You mean a bear run? Surely not accordingly. There will always be miners that will find profitable to mine. Someone may had a $10,000 equivalent daily income and with the decline he'd earn $7,000. If we assume that he spends $5,000 for electricity everyday, he'll find it profitable to continue mining. But, my thoughts are that this is a minority of mining farms.

I don't believe that the security of a payment system has to do anything with its market value.

That phrase is how I'd characterize the cryptocurrency industry in the last 2-3 years. There is, indeed, manipulation, but that affects directly the supply and demand. If we had 100 people wanting to buy 100 BTC for $1 each and suddenly, 300 newcomers realized that they can get rich from this thing, it'd increase the demand, whether they'd have been manipulated or not.