If the current Bitcoin miners lose 99.99% of their hash rate, mining the next block would take a lot more than the average set of 6 per hour (10 minutes). Is the mining difficulty somehow readjusted in order to face this potential problem?
According to the Bitcoin Wiki, the difficulty is only adjusted every 2016 blocks.
The difficulty of the mathematical problem is automatically adjusted by the network, such that it targets a goal of solving an average of 6 blocks per hour. Every 2016 blocks (solved in about two weeks), all Bitcoin clients compare the actual number created with this goal and modify the target by the percentage that it varied. The network comes to a consensus and automatically increases (or decreases) the difficulty of generating blocks.
However, Bitcoin Cash has a solution to the problem.
If less than 6 blocks have been mined in the past 12 hours. It becomes 20% easier to mine blocks.