Looks pretty stable to me:
I think that is a misleading image - it only goes to 5th November - remember we have ‘t seen a difficulty adjustment for BTC since the massive rise in BCH started. However, if you look on sites like bitcoinwisdom, you will see that a significant adjustment is coming (albeit very slowly!) and block times are well below the target of 10 minutes.
I think the OP raises a good question which I have been wondering about as well. The 2,016 block / “two week” gap on difficulty adjustment works fine with increasing hashrate or small reductions but it doesn’t cope well with a significant drop. Bitcoinwisdom currently forecasts the next adjustment is nearly 20 days away.