I'm sorry but that is one of the most short-sighted thing I've seen in last ... 20 years! So according to what you say it is better to paralize dns resolving in whole network rather than in case of "server down" situation just let ads trough for time needed for fix. And I'm not even saying about second Pihole for redundancy (which is naturaly first thought - or at least should be), like somone said there are situations where you are forced to give second DNS which makes Pihole useless because like you said yourself there is no fallbacks but alternatives! Second DNS should not be just a feature for voting - it just MUST be there!!!