You'd think that you'd make your healthcheck systems scale proportionately to the number of systems you've got last reporting active. That seems like a good thing to do.
Personally, I'm not sure this would result in the remaining systems getting healthchecked more often, though. How would that work?
Let's say: you've got 1m servers and (say) 24 healthcheck system. They each send out one service API probe every, oh, 15 seconds, for something on the severely high side of things, with a 3 second timeout. If you suddenly only have 500k servers due to a network partiation, this doesn't cause healthcheck to occur at 2x against the backend services. Infrastructure isn't built this way, because if it was, the healthcheck would be meaningless.
Very different than what the client would see on the service facing side. Even then, it's going to cause 2-3x load to all the systems due to 2x the traffic to each system (assuming it was an extremely homogenous system) and cause things to be slow, and you'd have seen a different failure mode.