A hardfork does not require 17/20 in a strict sense: Yes, it is a custom to enact them at that point, and generally respected as a means to avoid a blockchain split. However it is possible to hard fork that meta-rule itself, at any support level.

That would likely result in a split, no matter what subchain is longer, but only one of the sides owns the branding, has the exchange listings, and has the capacity to support core development.