If all eureka instances go away and then eureka fall back cluster is not accessible because of network  issues- the number of instances registered with eureka at startup will be zero.

This would mean the threshold for renewals will be zero which would mean the preservation mode will be disabled. It is better to enable the preservation mode here (will be valid for about 15 mins) until the threshold is updated.
Bump up revision number.
Bump up revision number - one more time.