if no one mines anymore (because there's a sudden alien attack centered right on the asics, because they built them with a material that will self-destruct on a certain date etc. etc.) the difficulty will drop drastically in a short time. at that point mining will resume Because at least I will start mining with CPU like Satoshi
If that apocalyptic attack happens exactly after the difficulty adjustment and it wipes out let's say 90% of the hash rate prepare for months of pain while we mine enough blocks to reach a new difficulty adjustment, the last major downturn in hash rate which was gradual and only 27% ended with a full week on top of the normal 14 days period.
If all asics disappear it will take just a few centuries to mine with a with cpus the required blocks. But I guess some will wait!