Oh nice, very helpful. The sibling thread mentions that this problem is specific to "ARTCCs", which are not the towers at airports but rather central hubs:

  Between the sectors administered by [smaller towers] are 20 contiguous areas of US airspace above 18,000 feet, each managed by an Air Route Traffic Control Center (ARTCC)...
  A flight is handed off from one Center to another until it descends near its destination, when control is transferred to the TRACON serving the destination, and ultimately to the tower controller serving the airport.

Wikipedia mentions they in particular use special "very high frequency aviation bands" (around "AM 118-137 MHz"), which is presumably why they're all linked together. It also mentions sharing systems, but no clear specifics that I can find. Certainly doesn't disprove the botched rollout theory, tho...

https://en.wikipedia.org/wiki/Area_control_center

EDIT: someone below posted this, seems relevant -- the 'Radar multiplexer pipeline' that's on a "fast track" and an "aggressive track": https://www.sunhillo.com/wp-content/uploads/2025/04/SUN3125-... TL;DR: upgrading radio systems from old analog tech to new digital tech, I think?