If I recall right, PortlandHODL was bragging about running 96 Core nodes back when he was laughing in that now-famous space where he DDoSed Knots nodes over Comsat. Have you considered that? Maybe the reason Core’s node count didn’t dip is because a bunch of spam apologists are propping it up with their own Sybil attack.
Discussion
I did consider that. If Core fans were spinning up nodes in response to an ongoing decline in organic users, I suspect it would appear as jerky waves, a sort of sawtooth pattern. But in fact Core's usage line became flat after the initial wave of dropouts, while Knots kept rising; that seems like a weird, inorganic pattern.
Judging by written testimonials I see everywhere, a lot of people that never ran nodes are running Knots now. The net sales of companies like Start9 also suggest that to be true.