if mining Bitcoin ever becomes unprofitable for miners, then we can rest assured the blockchain will fail.

From my understanding, Bitcoin's automatic difficulty regulation kind of handles this. If we assume that nobody will mine if it's not profitable, the mining difficulty will simply decrease when the number of miners decrease, thus increasing the profitability for those still mining. If Bitcoin's mining difficulty is updated about every two weeks, then mining would go back to being barely profitable every two weeks. If on the other hand, people keep mining even when it's not profitable in the hope that the price of Bitcoin will increase in the future, then I guess there isn't even a problem (yet). The only problem I see is if people mine unprofitably for too long and suddenly stop mining at the same time. Then the difficulty would remain high until the new difficulty update, but since the current difficulty is based on the previous period's time, the blockchain might be super slow for a long time, because the few miners take too long to generate enough blocks to reach the new update. Also, since the difficulty can only be adjusted by a certain amount for each period, the difficulty decrease might not be able to keep up with the decreasing amount of miners. I don't think this will kill blockchain, but it may give long periods of slowness.