Of course they know how many patients they will lose to a sufficient extent..and it doesn't need to be precise. If the facility is already struggling and is 70-80% Medicare/Medicaid, any distruption at all is enough.
Future prognostication of use based on past trends is pretty straightforward in the medical world.
Furthermore, you’re ignoring the fact that in many states (including and especially NC), almost EVERY practice and hospital in rural areas is owned by the same conglomerates. For example, you would have to look pretty hard to find a hospital or major practice east of I95 not owned by UNC Health or Vidant (formerly ECU Health). The same is true in the Triangle of UNC/Duke/Wake Med. Within an hour of Greensboro its all Moses Cone/Wake Forest. Everything west of GSO is Atrium Health.
They all know minute by minute what each practice, provider, and hospital is doing, where the patients come from, how they pay, and what their expected usage is.